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Complete Blood Count

Complete blood count

Definition

A complete blood count (CBC) is a series of tests used to evaluate the composition and concentration of the cellular components of blood. It consists of the following tests: red blood cell (RBC) count, white blood cell (WBC) count, and platelet count; measurement of hemoglobin and mean red cell volume; classification of white blood cells (WBC differential); and calculation of hematocrit and red blood cell indices . The hematocrit is the percentage of blood by volume that is occupied by the red cells (i.e., the packed red cell volume). Red blood cell indices are calculations derived from the red blood cell count, hemoglobin, and hematocrit that aid in the diagnosis and classification of anemia.


Purpose

The CBC provides valuable information about the blood and to some extent the bone marrow, which is the blood-forming tissue. The CBC is used for the following purposes:

  • as a preoperative test to ensure both adequate oxygen carrying capacity and hemostasis
  • to identify persons who may have an infection
  • to diagnose anemia
  • to identify acute and chronic illness, bleeding tendencies, and white blood cell disorders such as leukemia
  • to monitor treatment for anemia and other blood diseases
  • to determine the effects of chemotherapy and radiation therapy on blood cell production

Precautions

The CBC requires a sample of blood collected from a vein. The nurse or phlebotomist inserting the needle should clean the skin first. The tourniquet should be removed from the arm as soon as the blood flows. If a fingerstick is used to collect the blood, care must be taken to wipe away the first drop, and not to squeeze the finger excessively as this causes the blood to be diluted by tissue fluid. Many drugs affect the results by causing increased or decreased RBC, WBC, and/or platelet production. Medications should be taken into account when interpreting results.


Description

The CBC is commonly performed on an automated hematology analyzer using well mixed whole blood that is added to a chemical called EDTA to prevent clotting. A CBC is a group of tests used to quantify the number of RBCs, WBCs, and platelets, provide information about their size and shape, measure the hemoblobin content of RBCs, determine the percentage and absolute number of the five white blood cell types, and identify early and abnormal blood cells. These tests are performed simultaneously, (usually in less than one minute), using an automated hematology analyzer. When the performance limit of the automated hematology analyzer is exceeded, sample dilution or pretreatment, manual smear review, or manual cell counts may be required. Each laboratory has established rules for determining the need for manual smear review based upon specific CBC parameters. For example, a manual differential is always performed when nucleated immature red blood cells are found on an electronic cell count.


Electronic cell counting

Electronic blood cell counting is based upon the principle of impedance (i.e., resistance to current flow). Some hematology analyzers combine impedance counting with light scattering to measure platelets. A small sample of the blood is aspirated into a chamber (the WBC counting bath) and diluted with a balanced isotonic saline solution that is free of particles. The diluted blood sample is split into two parts, one for counting RBCs and platelets and the other for counting WBCs. The RBC portion is transferred to the RBC/platelet counting bath where it is diluted further. The other portion remains in the WBC bath and a detergent (lysing agent) is added to destroy (hemolyze) the red blood cells. A small portion of the diluted fluid in each bath is allowed to flow past a small aperture. An electrical current is produced in each aperture by two electrodes, one on the inside and the other on the outside of the aperture. The saline solution is responsible for conducting current between the electrodes. The cells move through the aperture one at a time. When a cell enters the aperture, it displaces a volume of electrolyte equal to its size. The cell acts as an electrical resistor, and impedes the flow of current. This produces a voltage pulse, the magnitude of which is proportional to the size of the cell. Instrument electronics are adjusted to discriminate voltage pulses produced by different cells. These adjustments are called thresholds. For example, the threshold for counting a RBC is equivalent to a cell volume of 36 femtoliters or higher. Voltage pulses that are equivalent to volumes of 220 femtoliters are counted as platelets. This process is repeated two more times so that the RBC, WBC, and platelet counts are performed in triplicate. Each time frame for counting is several seconds and many thousands of cells are counted. The computer processes the counting data first by determining the agreement between the three counts. If acceptable criteria are met, the counts are accepted and used to calculate the result.

The hemoglobin concentration is measured optically using the solution in the WBC bath. The lysing agent contains potassium cyanide that reacts with the hemoglobin to form cyanmethemoglobin. The optical density of the cyanmethemoglobin is proportional to hemoglobin concentration.

The voltage pulses produced by the white blood cells depend upon the size of the cell and its nuclear density. Therefore, the pulses may be analyzed to differentiate between the types of WBCs found. For example, lymphocytes are the smallest WBCs and comprise the lower end of the size scale. Monocytes, prolymphocytes, and immature granulocytes comprise the central area of the WBC histogram, and mature granulocytes comprise the upper end. In addition to cell sizing, automated instruments may use any of three other methods to distinguish between subpopulations. These are radio frequency conductance, forward and angular light scattering, and fluorescent staining.


Red blood cell count

The red cells, the most numerous of the cellular elements, carry oxygen from the lungs to the body's tissues. They are released from the bone marrow into the blood in an immature form called the reticulocyte that still retains much of the cellular RNA needed for hemoglobin production. Reticulocytes may be counted on some automated analyzers and are an index to recovery from anemia. The average life span of RBCs in the circulation is approximately 120 days.

The red blood cell (RBC) count determines the total number of red cells (erythrocytes) in a sample of blood. Most anemias are associated with a low RBC count, hemoglobin, and hematocrit. Common causes include excessive bleeding; a deficiency of iron, vitamin B12, or folic acid; destruction of red cells by antibodies or mechanical trauma; bone marrow malignancy and fibrosis; and structurally abnormal hemoglobin. The RBC count is also decreased due to cancer, kidney diseases, and excessive IV fluids. An elevated RBC count may be caused by dehydration, hypoxia (decreased oxygen), or a disease called polycythemia vera. Hypoxia may result from high altitudes, chronic obstructive lung diseases, and congestive heart failure.


Hematocrit and cell indices

The hematocrit is a test that measures the volume of blood in percent that is comprised of the red blood cells. Automated cell counters calculate the hematocrit by multiplying the RBC count by the mean red cell volume. A decrease in the number or size of red cells also decreases the amount of space they occupy, resulting in a lower hematocrit. Conversely, an increase in the number or size of red cells increases the amount of space they occupy, resulting in a higher hematocrit. Thalassemia minor, a genetic cause of anemia, is an exception in that it usually causes an increase in the number of red blood cells, but because they are small, it results in a decreased hematocrit.

The three main RBC indices are used to determine the average size and hemoglobin content of the RBCs and they help determine the cause of anemia. The three indices are described below:

  • Mean corpuscular volume (MCV)the average size of the red blood cells expressed in femtoliters. MCV is calculated by dividing the hematocrit (as percent) by the RBC count in millions per microliter of blood, then multiplying by 10.
  • Mean corpuscular hemoglobin (MCH)the average amount of hemoglobin inside an RBC expressed in picograms. The MCH is calculated by dividing the hemoglobin concentration in grams per deciliter by the RBC count in millions per microliter, then multiplying by 10.
  • Mean corpuscular hemoglobin concentration (MCHC)the average concentration of hemoglobin in the RBCs expressed in percent. It is calculated by dividing the hemoglobin in grams per deciliter by the hematocrit, then multiplying by 100.

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic. Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases including iron deficiency anemia, thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of macrocytic anemia are vitamin B12 deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g., malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells is normal.


White blood cell count

The majority of CBCs include both a WBC count and an automated differential. A differential determines the percentage of each of the five types of mature white blood cells. An elevated WBC count occurs in infection, allergy, systemic illness, inflammation, tissue injury, and leukemia. A low WBC count may occur in some viral infections, immunodeficiency states, and bone marrow failure. The WBC count provides clues about certain illnesses, and helps physicians monitor a patient's recovery from others. The differential will reveal which WBC types are affected most. For example, an elevated WBC count with an absolute increase in lymphocytes having an atypical appearance is most often caused by infectious mononucleosis. The differential will also identify early WBCs that may be reactive (e.g., a response to acute infection) or the result of a leukemia.

When the electronic WBC count is abnormal or a cell population is flagged, meaning that one or more of the results is atypical, a manual differential is performed. In that case, a wedge smear is prepared. This is done by placing a drop of blood on a glass slide, and using a second slide to pull the blood over the first slide's surface. The smear is air dried, then stained with Wright stain and examined under a microscope using oil immersion (1000x magnification). One hundred white cells are counted and identified as either neutrophils, lymphocytes, monocytes, eosinophils, or basophils based on the shape and appearance of the nucleus, the color of cytoplasm, and the presence and color of granules. The purpose is to determine if these cells are present in a normal distribution, or if one cell type is increased or decreased. Any atypical or immature cells also are counted.

In addition to determining the percentage of each mature white blood cell, the following tests are performed as part of the differential:

  • Evaluation of RBC morphology is performed. This includes grading of the variation in RBC size (anisocytosis) and shape (poikioocytosis); reporting the type and number of any abnormal RBCs such as target cells, sickle cells, stippled cells, etc.; reporting the presence of immature RBCs (polychromasia); and counting the number of nucleated RBCs per 100 WBCs.
  • An estimate of the WBC count is made and compared to the automated or chamber WBC count. An estimate of the platelet count is made and compared to the automated or chamber platelet count. Abnormal platelets such as clumped platelets or excessively large platelets are noted on the report.
  • Any immature white blood cells are included in the differential count of 100 cells, and any inclusions or abnormalities of the WBCs are reported.

WBCs consist of two main subpopulations, the mononuclear cells and the granulocytic cells. Mononuclear cells include lymphocytes and monocytes. Granulocytes include neutropohils (also called polymorphonuclear leukocytes or segmented neutrophils), eosinophils, and basophils. Each cell type is described below:

  • Neutrophils are normally the most abundant WBCs. They measure 1216 μm in diameter. The nucleus stains dark purple-blue, and is divided into several lobes (usually three or four) consisting of dense chromatin. A neutrophil just before the final stage of maturation will have an unsegmented nucleus in the shape of a band. These band neutrophils may be counted along with mature neutrophils or as a separate category. The cytoplasm of a neutrophil contains both primary (azurophilic) and secondary (specific) granules. The secondary granules are lilac in color and are more abundant, almost covering the pink cytoplasm. Neutrophils are phagocytic (able to engulf objects) cells and facilitate removal of bacteria and antibody-coated antigens. The neutrophilic granules are rich in peroxidase, and aid the cell in destroying bacteria and other ingested cells.
  • Eosinophils are 1416 μm in diameter and contain a blue nucleus that is segmented into two distinct lobes. The cytoplasm is filled with large refractile orange-red granules. The granules contain peroxidase, hydrolases, and basic proteins that aid in the destruction of phagocytized cells. Eosinophils are increased in allergic reactions and parasitic infections.
  • Basophils, like eosinophils, are 1416 μm in diameter and have a blue nucleus that is bilobed. The cytoplasm of the basophil is filled with large dark blue-black granules that may obscure the nucleus. These contain large amounts of histamine, heparin, and acid mucopolysaccharides. Basophils mediate the allergic response by releasing histamine.
  • Lymphocytes are the second most abundant WBCs. They may be small (79 μm in diameter) or large (1216 μm in diameter). The nucleus is dark blue and is nearly round or slightly indented and the chromatin is clumped and very dense. The cytoplasm is medium blue and usually agranular. An occasional lymphocyte will have a few azurophilic granules in the cytoplasm. Lymphocytes originate in the lymphoid tissues and are not phagocytic. They are responsible for initiating and regulating the immune response by the production of antibodies and cytokines.
  • Monocytes are the largest WBCs, measuring 1420 μm in diameter. They have a large irregularly shaped and folded blue nucleus with chromatin that is less dense than other WBCs. The cytoplasm is gray-blue, and is filled with fine dust-like lilac colored granules. Monocytes are phagocytic cells that process and present antigens to lymphocytes, an event required for lymphocyte activation.

Platelet count

Platelets are disk-shaped structures formed by the detachment of cytoplasm from megakaryocytes. They aid in the coagulation process by attaching or adhering to the walls of injured blood vessels, where they stick together to form the initial platelet plug. A low platelet count may occur in patients with AIDS, viral infections, lymphoma, and lupus erythematosus, or in patients taking certain drugs, most notably quinine and quinidine. Decreased platelet production is also a cause of thrombocytopenia, and may be due to aplastic anemia, leukemia, lymphoma, or bone marrow fibrosis. A low platelet count can occur due to increased destruction. This can result from antibody production that is often drug-induced (heparin treatment being a prominent cause). Increased destruction also results from autoantibody production as occurs in idiopathic thrombocytopenic purpura (ITP) and thrombotic episodes that consume platelets such as occur in thrombotic thrombocytopenic purpura (TTP), disseminated intravascular coagulation (DIC), and hemolytic-uremic syndrome (HUS). Inherited (congenital) thrombocytopenia can be caused by Glanzmann's thrombasthenia, Fanconi syndrome, and Wiskott-Aldrich syndrome.

Thrombocytosis, an increased platelet count, is most often caused by a reaction to injury or inflammation. In these cases the platelet count increases transiently and is usually within the range of 400,000800,000 per microliter. Persistent or higher counts are usually associated with myeloproliferative disease (malignant disease involving blood forming cells) such as chronic granulocytic (myelogenous) leukemia, polycythemia vera, or primary (essential) thrombocythemia.

The platelet count is most often measured by impedance counting but is performed manually when the platelet count is very low, platelet clumping is observed, or abnormally large (giant) platelets are present. Often these abnormalities and others such as cryoglobulinemia, cell fragmentation (hemolysis), and microcytic RBCs are signaled by abnormal RBC and platelet indices and abnormal population flags. An abnormal mean platelet volume or platelet histogram indicates that morphological platelet abnormalities are present and the platelets should be observed from a stained blood film to characterize the abnormality. The platelet count can be estimated using the Wright-stained blood smear used for a differential WBC count by multiplying the average number of platelets per oil immersion field by 20,000. Platelet estimates should correlate with actual counts. When they disagree, the platelet count should be repeated and a manual count performed if necessary.


Preparation

The CBC does not require fasting or any special preparation.

Aftercare

Discomfort or bruising may occur at the puncture site. Applying pressure to the puncture site until the bleeding stops helps to reduce bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated by resting awhile.


Risks

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.


Normal results

CBC values vary by age and sex. Normal values are ultimately determined by the laboratory performing the test. As a guide, the normal values for men and nonpregnant women are as follows:

  • WBCs: 4,50011,000 per microliter for women and men, with neutrophils representing 5070%, lymphocytes 2535%, monocytes 46%, eosinophils 13%, basophils 0.41%, and bands 05%.
  • RBCs: 4.25.0 million per microliter for women; 4.56.2 million per microliter for men.
  • Hemoglobin: 1215 g/dL for women; 13.617.2 g/dL for men.
  • Hematocrit: 3547% for women; 4252% for men.
  • Platelets: 150,000 and 350,000 per microliter.
  • Reticulocyte count: 0.51.5%.

Normal adult results for red blood cell indices are as follows:

  • MCV: 8098 fl (femtoliters)
  • MCHC: 3236%
  • MCH: 2731 pg (picograms)
  • RDW: 11.514.5%

In addition to normal values, critical values (alert, panic values) are established for hemoglobin (and hematocrit), WBC count, and platelet count. Precipitously low hemoglobin is associated with hypoxia that can have life-threatening complications. Extremely low WBCs indicates an inability to fight infection and a high risk of sepsis. A severely reduced platelet count predisposes the patient to spontaneous internal bleeding. Representative critical values are shown below.

  • Hemoglobin: less than 5.0 g/dL
  • Hematocrit: less than 15%
  • Platelet count: less than 30,000 per microliter
  • WBC count: less than 2,500 per microliter and greater than 30,000 per microliter

Abnormal blood count results are seen in a variety of conditions. One of the most common is anemia, which is characterized by a low RBC count, hemoglobin, and hematocrit. The category into which a person's anemia is placed is in part based upon the red blood cell indices provided. The indices provide a significant clue as to the cause of the anemia, but further testing is needed to confirm a specific diagnosis. The most common causes of macrocytic anemia (high MCV) are vitamin B12 and folic acid deficiencies. Lack of iron in the diet, thalassemia (a type of hereditary anemia), and chronic illness are the most common causes of microcytic anemia (low MCV). Normocytic anemia (normal MCV) can be caused by kidney and liver disease, bone marrow disorders, leukemia, excessive bleeding, or hemolysis of the red blood cells. Iron deficiency and thalassemia are the most common causes of hypochromic anemia (low MCHC). Normocytic anemias are usually also normochromic and share the same causes. The red cell distribution width (RDW) is increased in anemias caused by deficiencies of iron, vitamin B12, or folic acid. Abnormal hemoglobins, such as in sickle cell anemia, can change the shape of red blood cells as well as cause them to hemolyze, or rupture. The abnormal shape and the cell fragments resulting from hemolysis increase the RDW. Conditions that cause more immature cells to be released into the bloodstream, such as severe blood loss, will increase the RDW. The larger size of immature cells creates a distinct size variation.

Infections and leukemias are associated with increased numbers of WBCs. Increases or decreases in the percentage of each white cell can be associated with a number of diseases or conditions, including cancer, leukemia, anemia, multiple sclerosis, allergies, parasitic and viral diseases, infections, and tissue damage.

Resources

books

Chernecky, Cynthia C. and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders, 2001.

Henry, John B. Clinical Diagnosis and Management by Laboratory Methods. Philadelphia: W. B. Saunders, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2000.


Victoria E. DeMoranville Mark A. Best

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Complete Blood Count

COMPLETE BLOOD COUNT

The clinical laboratory test that evaluates the three main cellular components of peripheral blood (red cells, white cells, and platelets) is called the "complete blood count" (CBC). It is used commonly to assess whether a patient is anemic (low red cell count), has an infection (increased white blood cells), or has abnormal blood coagulation (platelet levels). The CBC examines the total number of red blood cells (RBC) and the RBC indices, including: the mean corpuscular volume (MCV); the concentration of hemoglobin, measured by the mean corpuscular hemoglobin (MCH) and its concentration (MCHC); and the hematocrit, which is the mean packed-cell volume of red cells. The total white blood cell (leukocyte) count, the various types of leukocytes (lymphocytes, monocytes, neutrophils, eosinophils, and basophils), and platelets are also measured.

Jonathan R. Keller

Mariaestela Ortiz

(see also: Hematocrit; Hemoglobin; Laboratory Services )

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Complete Blood Count

Complete Blood Count

Definition

A complete blood count (CBC) is a series of tests used to evaluate the composition and concentration of the cellular components of blood. It consists of the following tests: red blood cell (RBC) count, white blood cell (WBC) count, and platelet count; measurement of hemoglobin and mean red cell volume; classification of white blood cells (WBC differential); and calculation of hematocrit and red blood cell indices. The hematocrit is the percentage of blood by volume that is occupied by the red cells (i.e., the packed red cell volume). Red blood cell indices are calculations derived from the red blood cell count, hemoglobin and hematocrit that aid in the diagnosis and classification of anemia.

Purpose

The CBC provides valuable information about the blood and to some extent the bone marrow which is the blood-forming tissue. The CBC is used for the following purposes:

  • As a preoperative test to ensure both adequate oxygen carrying capacity and hemostasis.
  • To identify persons who may have an infection.
  • To diagnose anemia.
  • To identify acute and chronic illness, bleeding tendencies, and white blood cell disorders such as leukemia.
  • To monitor treatment for anemia and other blood diseases.
  • To determine the effects of chemotherapy and radiation therapy on blood cell production.

Precautions

The CBC requires a sample of blood collected from a vein. The nurse or phlebotomist performing the venipuncture should observe universal precautions for the prevention of transmission of bloodborne pathogens. Thecollection tubemustbe filledcompletely, as under-filling increases the anticoagulant (EDTA) to blood ratio, which will crenate red blood cells. The tourniquet should be removed from the arm as soon as the blood flows to prevent hemoconcentration. If a fingerstick is used to collect the blood, care must be taken to wipe away the first drop, and not to squeeze the finger excessively as this causes the blood to be diluted by tissue fluid. The tests should be performed within four hours of collection or the sample must be refrigerated. Samples stored at 35-46 °F (2-8°C) may be measured for up to 18 hours. Samples must be thoroughly mixed prior to measurement. Many drugs affect the results by causing increased or decreased RBC, WBC, and/or platelet production. Medications should be taken into account when interpreting results.

Description

The CBC is commonly performed on an automated hematology analyzer using well mixed whole blood anticoagulated with EDTA. A CBC is a group of tests used to quantify the number of RBCs, WBCs, and platelets, provide information about their size and shape, measure the hemoblobin content of RBCs, determine the percentage and absolute number of the five white blood cell types, and identify early and abnormal blood cells. These tests are performed simultaneously, (usually in less than one minute), using an automated hematology analyzer. When the performance limit of the automated hematology analyzer is exceeded, sample dilution or pretreatment, manual smear review, or manual cell counts may be required. Such conditions include very low or elevated cell counts, and the presence of cold agglutinins, lipemia, and cell fragments. For example, a manual WBC count may be performed when the automated WBC count is below 500 per microliter. A manual microscopic evaluation of a stained blood film is performed when an abnormal cell population is encountered. Each laboratory has established rules for determining the need for manual smear review based upon specific CBC parameters. For example, a manual differential is always performed when nucleated red blood cells are found on an electronic cell count.

Electronic cell counting

Electronic blood cell counting is based upon the principle of impedance (i.e., resistance to current flow). Some hematology analyzers combine impedance counting with light scattering to measure platelets. A small sample of the blood is aspirated into a chamber (the WBC counting bath) and diluted with a balanced isotonic saline solution that is free of particles. The diluted blood sample is split into two parts, one for counting RBCs and platelets and the other for counting WBCs. The RBC portion is transferred to the RBC/platelet counting bath where it is diluted further. The other portion remains in the WBC bath and a detergent (lysing agent) is added to destroy (hemolyze) the red blood cells. A small portion of the diluted fluid in each bath is allowed to flow past a small aperture. An electrical current is produced in each aperture by two electrodes, one on the inside and the other on the outside of the aperture. The saline solution is responsible for conducting current between the electrodes. The cells move through the aperture one at a time. When a cell enters the aperture, it displaces a volume of electrolyte equal to its size. The cell acts as an electrical resistor, and impedes the flow of current. This produces a voltage pulse the magnitude of which is proportional to the size of the cell. Instrument electronics are adjusted to discriminate voltage pulses produced by different cells. These adjustments are called thresholds. For example, the threshold for counting a RBC is equivalent to a cell volume of 36 femtoliters or higher. Voltage pulses that are equivalent to volumes of 2-20 femtoliters are counted as platelets. This process is repeated two more times so that the RBC, WBC, and platelet counts are performed in triplicate. Each time frame for counting is several seconds and many thousands of cells are counted. The computer processes the counting data first by determining the agreement between the three counts. If acceptable criteria are met the counts are accepted and used to calculate the result. The computer mathematically corrects the count for the random chance of two cells entering the aperture simultaneously. The voltage pulses for each cell type are sorted and displayed. The RBC and platelet sizes are plotted as a histogram, and the WBC sizes are plotted as a scattergram. This process produces cell counts with coefficients of variation that are on the order of tenfold lower than can be achieved by manual cell counting.

The hemoglobin concentration is measured optically using the solution in the WBC bath. The lysing agent contains potassium cyanide that reacts with the hemoglobin to form cyanmethemoglobin. The optical density of the cyanmethemoglobin is proportional to hemoglobin concentration. Source light from a small tungsten lamp or an LED that produces monochromatic light is directed through the sample contained in a small tube behind the bath. An interference filter on the other side of the tube transmits unabsorbed monochromatic light (e.g., 525 nm) to a photodiode. The photodiode current is proportional to the light it receives. This electronic signal is converted to an inverse log voltage that is proportional to the optical density of the solution. The optical density reading for the diluent is subtracted from the sample and the value is multiplied by a calibration factor (determined by measuring a calibrating solution) in order to calculate hemoglobin concentration.

The voltage pulses produced by the white blood cells depend upon the size of the cell and its nuclear density. Therefore, the pulses may be analyzed to differentiate between the types of WBCs found. For example, lymphocytes are the smallest WBCs and comprise the lower end of the size scale. Monocytes, prolymphocytes, and immature granulocytes comprise the central area of the WBC histogram, and mature granulocytes comprise the upper end. In addition to cell sizing, automated instruments may use any of three other methods to distinguish between subpopulations. These are radio frequency conductance, forward and angular light scattering, and fluorescent staining.

Red blood cell count

The red cells, the most numerous of the cellular elements, carry oxygen from the lungs to the body's tissues. They are released from the bone marrow into the blood in an immature form called the reticulocyte that still retains much of the cellular RNA needed for hemoglobin production. Reticulocytes may be counted on some automated analyzers and are an index to recovery from anemia. The average life span of RBCs in the circulation is approximately 110 days.

The red blood cell (RBC) count determines the total number of red cells (erythrocytes) in a sample of blood. Most anemias are associated with a low RBC count, hemoglobin, and hematocrit. Common causes include excessive bleeding; a deficiency of iron, vitamin B12, or folic acid; destruction of red cells by antibodies or mechanical trauma; bone marrow malignancy and fibrosis; and structurally abnormal hemoglobin. The RBC count is also decreased due to cancer, kidney diseases, and excessive IV fluids. An elevated RBC count may be caused by dehydration, hypoxia, or polycythemia vera. Hypoxia may result from high altitudes, chronic obstructive lung diseases, and congestive heart failure.

Hematocrit and cell indices

The hematocrit is a test that measures the volume of blood in percent that is comprised of the red blood cells. Automated cell counters calculate the hematocrit by multiplying the RBC count by the mean red cell volume (see MCV below). A decrease in the number or size of red cells also decreases the amount of space they occupy, resulting in a lower hematocrit. Conversely, an increase in the number or size of red cells increases the amount of space they occupy, resulting in a higher hematocrit. Thalassemia minor is an exception in that it usually causes an increase in the number of red blood cells, but because they are small, it results in a decreased hematocrit.

The three RBC indices are used to determine the average size and hemoglobin content of the RBCs and they help determine the cause of anemia. The three indices are described below:

  • Mean corpuscular volume (MCV)—the average size of the red blood cells expressed in femtoliters. MCV is calculated by dividing the hematocrit (as percent) by the RBC count in millions per microliter of blood, then multiplying by 10.
  • Mean corpuscular hemoglobin (MCH)—the average amount of hemoglobin inside an RBC expressed in picograms. The MCH is calculated by dividing the hemoglobin concentration in grams per deciliter by the RBC count in millions per microliter, then multiplying by 10.
  • Mean corpuscular hemoglobin concentration (MCHC)—the average concentration of hemoglobin in the RBCs expressed in percent. It is calculated by dividing the hemoglobin in grams per deciliter by the hematocrit, then multiplying by 100.

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic. Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases including iron deficiency anemia, thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of marcocytic anemia are vitamin B12 deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g., malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells is normal.

White blood cell count

The majority of CBCs include both a WBC count and an automated differential. A differential determines the percentage of each of the five types of mature white blood cells. An elevated WBC count occurs in infection, allergy, systemic illness, inflammation, tissue injury, and leukemia. A low WBC count may occur in some viral infections, immunodeficiency states, and bone marrow failure. The WBC count provides clues about certain illnesses, and helps physicians monitor a patient's recovery from others. The differential will reveal which WBC types are affected most. For example, an elevated WBC count with an absolute increase in lymphocytes having an atypical appearance is most often caused by infectious mononucleosis. The differential will also identify early WBCs which may be reactive (e.g., a response to acute infection) or the result of a leukemia.

When the electronic WBC count is abnormal or a cell population is flagged, meaning that one or more of the results is atypical, a manual differential is performed. In that case, a wedge smear is prepared. This is done by placing a drop of blood on a glass slide, and using a second slide to pull the blood over the first slide's surface. The smear is air dried, then stained with Wright stain and examined under a microscope using oil immersion (1000× magnification). One hundred white cells are counted and identified as either neutrophils, lymphocytes, monocytes, eosinophils, or basophils based on the shape and appearance of the nucleus, the color of cytoplasm, and the presence and color of granules. The purpose is to determine if these cells are present in a normal distribution, or if one cell type is increased or decreased. Any atypical or immature cells also are counted.

In addition to determining the percentage of each mature white blood cell, the following tests are performed as part of the differential:

  • Evaluation of RBC morphology is performed. This includes grading of the variation in RBC size (anisocytosis) and shape (poikilocytosis); reporting the type and number of any abnormal RBCs such as target cells, sickle cells, stippled cells, etc.; reporting the presence of immature RBCs (polychromasia); and counting the number of nucleated RBCs per 100 WBCs.
  • An estimate of the WBC count is made and compared to the automated or chamber WBC count. An estimate of the platelet count is made and compared to the automated or chamber platelet count. Abnormal platelets such as clumped platelets or excessively large platelets are noted on the report.
  • Any immature white blood cells are included in the differential count of 100 cells, and any inclusions or abnormalities of the WBCs are reported.

WBCs consist of two main subpopulations, the mononuclear cells and the granulocytic cells. Mononuclear cells include lymphocytes and monocytes. Granulocytes include neutrophils (also called polymorphonuclear leukocytes or segmented neutrophils), eosinophils, and basophils. Each cell type is described below:

  • Neutrophils are normally the most abundant WBCs. They measure 12-16 κm in diameter. The nucleus stains dark purple-blue, and is divided into several lobes (usually three or four) consisting of dense chromatin. A neutrophil just before the final stage of maturation will have an unsegmented nucleus in the shape of a band. These band neutrophils may be counted along with mature neutrophils or as a separate category. The cytoplasm of a neutrophil contains both primary (azurophilic) and secondary (specific) granules. The secondary granules are lilac in color and are more abundant, almost covering the pink cytoplasm. Neutrophils are phagocytic cells and facilitate removal of bacteria and antibody-coated antigens. The neutrophilic granules are rich in peroxidase, and aid the cell in destroying bacteria and other ingested cells.
  • Eosinophils are 14-16 κm in diameter and contain a blue nucleus that is segmented into two distinct lobes. The cytoplasm is filled with large refractile orange-red granules. The granules contain peroxidase, hydrolases, and basic proteins that aid in the destruction of phagocytized cells. Eosinophils are increased in allergic reactions and parasitic infections.
  • Basophils, like eosinophils, are 14-16 κm in diameter and have a blue nucleus that is bilobed. The cytoplasm of the basophil is filled with large dark blueblack granules that may obscure the nucleus. These contain large amounts of histamine, heparin, and acid mucopolysaccharides. Basophils mediate the allergic response by releasing histamine.
  • Lymphocytes are the second most abundant WBCs. They may be small (7-9 κm in diameter) or large (12-16 κm in diameter). The nucleus is dark blue and is nearly round or slightly indented and the chromatin is clumped and very dense. The cytoplasm is medium blue and usually agranular. An occasional lymphocyte will have a few azurophilic granules in the cytoplasm. Lymphocytes originate in the lymphoid tissues and are not phagocytic. They are responsible for initiating and regulating the immune response by the production of antibodies and cytokines.
  • Monocytes are the largest WBCs, measuring 14-20 κm in diameter. They have a large irregularly shaped and folded blue nucleus with chromatin that is less dense than other WBCs. The cytoplasm is gray-blue, and is filled with fine dust-like lilac colored granules. Monocytes are phagocytic cells that process and present antigens to lymphocytes, an event required for lymphocyte activation.

Platelet count

Platelets are disk-shaped structures formed by the detachment of cytoplasm from megakaryocytes. They aid in the coagulation process by attaching or adhering to the walls of injured blood vessels, where they stick together to form the initial platelet plug. A low platelet count may occur in patients with AIDS, viral infections, lymphoma, and lupus erythematosus, or in patients taking certain drugs, most notably quinine and quinidine. Decreased platelet production is also a cause of thrombocytopenia, and may be due to aplastic anemia, leukemia, lymphoma, or bone marrow fibrosis. A low platelet count can occur due to increased destruction. This can result from alloantibody production which is often drug-induced (heparin treatment being a prominent cause). Increased destruction also results from autoantibody production as occurs in idiopathic thrombocytopenic purpura (ITP) and thrombotic eipisodes that consume platelets such as occur in thrombotic thrombocytopenic purpura (TTP), disseminated intravascular coagulation (DIC), and hemolytic-uremic syndrome (HUS). Inherited (congenital) thrombocytopenia can be caused by Glanzmann's thrombasthenia, von Willebrand's disease, Fanconi syndrome, and Wiskott-Aldrich syndrome.

Thrombocytosis, an increased platelet count, is most often caused by a reaction to injury or inflammation. In these cases the platelet count increases transiently and is usually within the range of 400,000-800,000 per microliter. Persistent or higher counts are usually associated with myeloproliferative disease (malignant disease involving blood forming cells) such as chronic granulocytic (myelogenous) leukemia, polycythemia vera, or primary (essential) thrombocythemia.

The platelet count is most often measured by impedance counting but is performed manually when the platelet count is very low, platelet clumping is observed, or abnormally large (giant) platelets are present. Often these abnormalities and others such as cryoglobulinemia, cell fragmentation (hemolysis), and microcytic RBCs are signaled by abnormal RBC and platelet indices and abnormal population flags. An abnormal mean platelet volume or platelet histogram indicates that morphological platelet abnormalities are present and the platelets should be observed from a stained blood film to characterize the abnormality. The platelet count can be estimated using the Wrightstained blood smear used for a differential WBC count by multiplying the average number of platelets per oil immersion field by 20,000. Platelet estimates should correlate with actual counts. When they disagree, the platelet count should be repeated and a manual count performed if necessary.

Preparation

The CBC does not require fasting or any special preparation.

Aftercare

Discomfort or bruising may occur at the puncture site. Applying pressure to the puncture site until the bleeding stops helps to reduce bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated accordingly.

Complications

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.

Results

CBC values vary by age and sex. Normal values are ultimately determined by the laboratory performing the test. As a guide, the normal values for men and non-pregnant women are as follows:

  • WBCs: 4,500 to 11,000 per microliter for women and men, with neutrophils representing 50-70%, lymphocytes 25-35%, monocytes 4-6%, eosinophils 1-3%, basophils 0.4-1%, and bands 0-5%.
  • RBCs: 4.2 to 5.0 million per microliter for women; 4.5 to 6.2 million per microliter for men.
  • Hemoglobin: 12-15 g/dL for women; 13.6 to 17.2 g/dL for men.
  • Hematocrit: 35-47% for women; 42-52% for men.
  • Platelets: 150,000 and 350,000 per microliter.
  • Reticulocyte count: 0.5-1.5%.

Normal adult results for red blood cell indices are as follows:

  • MCV: 80-98 fl (femtoliters).
  • MCHC: 32-36%.
  • MCH: 27-31 pg (picograms).
  • RDW: 11.5-14.5%.

KEY TERMS

Anemia— A diminished oxygen carrying capacity caused by a decrease in the size, number, or function of red blood cells.

Capillaries— The smallest of the blood vessels that bring oxygenated blood to tissues.

EDTA— A colorless compound used to keep blood samples from clotting. Its chemical name is ethylene-diaminetetraacetic acid. EDTA functions by binding the calcium in the blood sample.

Hematocrit— The volume of blood occupied by the red blood cells expressed in percent.

Hemoglobin— A protein inside red blood cells that carries oxygen to body tissues.

Hypochromic— A descriptive term applied to a red blood cell with a decreased concentration of hemoglobin.

Macrocytic— A descriptive term applied to a larger than normal red blood cell.

Mean corpuscular hemoglobin (MCH)— A measurement of the average weight of hemoglobin in a red blood cell.

Mean corpuscular hemoglobin concentration (MCHC)— The measurement of the average concentration of hemoglobin in a red blood cell.

Mean corpuscular volume (MCV)— A measure of the average volume of a red blood cell.

Mean platelet volume (MPV)— A measure of the average volume of a platelet.

Microcytic— A descriptive term applied to a smaller than normal red blood cell.

Normochromic— A descriptive term applied to a red blood cell with a normal concentration of hemoglobin.

Normocytic— A descriptive term applied to a red blood cell of normal size.

Red blood cell indices— Measurements that describe the size and hemoglobin content of red blood cells.

Red cell distribution width (RDW)— A measure of the variation in size of red blood cells.

Thrombocyte— Another name for platelet.

Thrombocytopenia— An abnormally low platelet count.

Thrombocytosis— An abnormally high platelet count. It occurs in polycythemia vera and other disorders in which the bone marrow produces too many platelets.

In addition to normal values, critical values (alert, panic values) are established for hemoglobin (and hematocrit), WBC count, and platelet count. Precipitously low hemoglobin is associated with hypoxia that can have life-threatening complications. Extremely low WBCs indicates an inability to fight infection and a high risk of sepsis. A severely reduced platelet count predisposes the patient to spontaneous internal bleeding. Representative critical values are shown below.

  • Hemoglobin: less than 5.0 g/dL.
  • Hematocrit: less than 15%.
  • Platelet count: less than 30,000 per microliter.
  • WBC count: less than 2,500 per microliter and greater than 30,000 per microliter.

Abnormal blood count results are seen in a variety of conditions. One of the most common is anemia, which is characterized by a low RBC count, hemoglobin, and hematocrit. The category into which a person's anemia is placed is in part based upon the red blood cell indices provided. The indices provide a significant clue as to the cause of the anemia, but further testing is needed to confirm a specific diagnosis. The most common causes of macrocytic anemia (high MCV) are vitamin B12 and folic acid deficiencies. Lack of iron in the diet, thalassemia (a type of hereditary anemia), and chronic illness are the most common causes of microcytic anemia (low MCV). Normocytic anemia (normal MCV) can be caused by kidney and liver disease, bone marrow disorders, leukemia, excessive bleeding, or hemolysis of the red blood cells. Iron deficiency and thalassemia are the most common causes of hypochromic anemia (low MCHC). Normocytic anemias are usually also normochromic and share the same causes. The red cell distribution width (RDW) is increased in anemias caused by deficiencies of iron, vitamin B12, or folic acid. Abnormal hemoglobins, such as in sickle cell anemia, can change the shape of red blood cells as well as cause them to hemolyze. The abnormal shape and the cell fragments resulting from hemolysis increase the RDW. Conditions that cause more immature cells to be released into the bloodstream, such as severe blood loss, will increase the RDW. The larger size of immature cells creates a distinct size variation.

Infections and leukemias are associated with increased numbers of WBCs. Increases or decreases in the percentage of each white cell can be associated with a number of diseases or conditions, including cancer, leukemia, anemia, multiple sclerosis, allergies, parasitic and viral diseases, infections, and tissue damage.

Health care team roles

The CBC is ordered by a physician. A nurse or phlebotomist usually draws the blood to be used for the test. A clinical laboratory scientist/medical technologist or clinical laboratory technician/medical laboratory technician performs the test. The laboratory personnel are responsible for notifying the physician when a critical limit is encountered. A clinical pathologist may be consulted when abnormal findings that indicate a serious blood disease are noted, such as early cells or cells containing inclusions.

Resources

BOOKS

Chernecky, Cynthia C., and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures, 3rd ed. Philadelphia: W. B. Saunders Company, 2001.

Henry, John B. Clinical Diagnosis and Management by Laboratory Methods. Philadelphia: W. B. Saunders Company, 1996.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Merck Manual of Medical Information, edited by Robert Berkow, et al. Whitehouse Station, NJ: Merck Research Laboratories, 1997.

Mosby's Diagnostic and Laboratory Test Reference, edited by Kathleen Deska Pagana, and Timothy James Pagana. St. Louis: Mosby-Year Book, Inc., 1998.

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Complete Blood Count

Complete blood count

Definition

A complete blood count is not a single test, but a series of tests performed on a blood sample to screen for a variety of diseases and conditions.

Purpose

The complete blood count is a variety of different tests, and can provide a lot of information about an individual's health. Some doctors may order a complete blood count to get a baseline for a healthy individual against which later complete blood counts can be compared. It can also be ordered to check general health. In many cases, however, a complete blood count is performed to help gather information about what might be causing one or more symptoms that are difficult to diagnose on their own. Symptoms for which a complete blood count may be ordered include fatigue, bruising , infection, or weakness. A complete blood count may also be ordered as part of treatment and monitoring of a variety and diseases including cancer to help health professionals monitor the levels of certain types of cells.

Precautions

No special precautions are necessary for a complete blood count.

Description

When a complete blood count is performed a nurse takes a small amount of the patient's blood, usually through a vein in the arm. The nurse then labels the vial of blood with the patient's information and sends it to a laboratory for analysis. The laboratory technicians do a variety of tests on the blood and look at it under the microscope to gather information on a variety of attributes of the blood.

A complete blood count generally consists of ten different pieces of information about the make-up of a patient's blood. Each of these pieces of information can be of importance when diagnosing different diseases and conditions. The ten parts of a complete blood count are:

  • White blood cell count—The number of white blood cells present in a certain amount of a patient's blood.
  • White blood cell differential—The number of each type of the five major types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophiles.
  • Red blood cell count—The number of red blood cells present in a certain amount of a patient's blood.
  • Red cell distribution width—The variety seen in the width of the red blood cells present in the blood.
  • Hemoglobin—The amount of hemoglobin present in the blood.
  • Hematocrit—The percentage of the blood that is made up of red blood cells.
  • Platelet count—The number of platelets present in a certain amount of a patient's blood.
  • Mean corpuscular volume—The average size of the red blood cells present in a patient's blood.
  • Mean corpuscular hemoglobin—The average amount of hemoglobin inside each red blood cell.
  • Mean corpuscular hemoglobin concentration—The average concentration of hemoglobin inside each red blood cell.

Preparation

There are no specific patient preparations generally required for a complete blood count. The site from which blood is going to be drawn for the test is wiped with an alcohol wipe to sterilize the site before the blood is drawn.

Aftercare

No aftercare is generally required for a complete blood count. The site from which the blood is drawn is usually covered with a gauze pad held in place with an adhesive bandage.

QUESTIONS TO ASK YOUR DOCTOR

  • Why is a complete blood count indicated for me?
  • What results will indicate that there is a problem?
  • How soon can I expect the results of the test?
  • What is the next step if the results show there is a problem?

Complications

For most individuals the blood draw for the complete blood count is quick and relatively painless. In some cases repeated attempts may be necessary, especially in individuals who have had many blood draws over time. Seniors also may be more likely to have veins that require repeated sticking for success. Finger-stick blood collection methods are usually appropriate for complete blood counts, which may reduce the necessity of repeated attempts. In rare cases blood draw can lead to complications such as infection, bruising, or excessive bleeding.

Results

There are a variety of results that the complete blood count can produce. Raised or lowered levels of any of the types of cells can have health implications. Changes of cell width, size, and density can also be a sign of health concerns.

  • White blood cell count—A raised white blood cell count can indicate the presence of an infection, cancer, or other problems. A lowered white blood cell count may indicate bone marrow problems, and certain autoimmune conditions.
  • White blood cell differential—Increased or decreased numbers of certain types of white blood cells can indicate leukemia, infections, and allergic reactions.
  • Red blood cell count—An increase in red blood cells can indicate serious burns, dehydration, or diarrhea. Decreases in red blood cell count are a sign of anemia.
  • Red cell distribution width—Increased red cell distribution width can be a sign of certain types of anemia, and liver disease. Decreased red cell distribution width can indicate a variety of types of anemia.
  • Hemoglobin—Increased hemoglobin levels can indicate dehydration, diarrhea, or burns. Increased hemoglobin levels can indicate anemia.
  • Hematocrit—Increased hematocrit levels can be a sign of dehydration, diarrhea, or significant burns. Decreased levels are a sign of anemia.
  • Platelet count—Increases in platelet let count indicates that there may be increased chances of dangerous blood clots forming. Decreased platelet count can be cause serious bleeding complications. Decreased platelet count can be a sign of anemia, hypersplenism, leukemia, lupus, and a side effect of chemotherapy.
  • Mean corpuscular volume—Increased mean corpuscular volume can indicate folate or B12 deficiency. Decreased mean corpuscular volume can indicate thalassemia or iron deficiency.
  • Mean corpuscular hemoglobin—Increased mean corpuscular hemoglobin is a sign of B12 or folate deficiency. Increased levels can indicate thalassema or iron deficiency.
  • Mean corpuscular hemoglobin concentration—Increased mean corpuscular hemoglobin concentration is a sign of anemia. Decreased mean corpuscular hemoglobin concentration is a sign of iron deficiency, thalassema, and significant, chronic blood loss.

KEY TERMS

Phlebotomy —The drawing of blood.

Platelet —The smallest of the red blood cells that play an important role in blood clotting.

Caregiver concerns

A doctor orders a complete blood count after assessing the patient's symptoms. The blood draw for the test is performed by a nurse trained in phlebotomy (the drawing of blood). The complete blood count is actually performed by one or more laboratory technicians. The results are then recorded and sent back to the doctor so that he or she can determine what if any further testing or treatment is necessary.

Resources

books

Bain, Barbara J. Blood Cells: A Practical Guide, 4th Ed. Malden, MA: Blackwell, 2006.

Ciesla, Betty. Hematology in Practice. Philadelphia, PA: F.A. Davis Co., 2007.

Hoffman, Ronald, et al, eds. Hematology: Basic Principles and Practice, 4th Ed. Philadelphia, PA: Churchill Livingstone, 2005.

periodicals

Baer, Daniel M. “Answering your questions: manual cellcount controls.” Medical Laboratory Observer 38.6 (June 2006): 35–36.

“Blood cell counters.” Bioscience Technology 31.6 (June 2006): 29–30.

organizations

Leukemia and Lymphoma Society, 1311 Mamaroneck Avenue, Suite 130, White Plains, NY, 10605, 914-949-0084, http://www.leukemia-lymphoma.org/.

Robert Bockstiegel

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Complete Blood Count

Complete Blood Count

Definition
Purpose
Description
Precautions
Side effects
Interactions

Definition

A complete blood count (CBC) provides important information about the types and numbers of cells in the blood; in particular, information about red blood cells, white blood cells and platelets.

Purpose

The purpose of a CBC is to help physicians to diagnose conditions related to abnormalities in the blood such as infections and anemia.

Description

A complete blood count usually includes the following elements:

  • Red blood cell count (also called RBC or erythrocyte count)
  • Red blood cell indices - mean corpuscular volume (MCV, mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC)
  • Hemoglobin (also called Hgb)
  • Hematocrit (also called HCT)
  • White blood cell count (also called WBC or leukocyte count)
  • Platelet count (also called thrombocyte count)

Red blood cells (erythrocytes) transport oxygen between the lungs and cells throughout the rest of the body. They also transport carbon dioxide back to the lungs so it can be exhaled. A low red cell count may be due to anemia and cells in the body may not be getting the oxygen that they need. A red blood cell count that is abnormally high may be due to an uncommon condition called polycythemia.

White blood cells (leukocytes) protect the body against infection. When an infection develops, white blood cells attack and destroy the pathogen (bacteria,

WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?

  • A complete blood count is typically ordered by a family doctor, internist or surgeon but any physician may order one.
  • A blood sample is usually obtained by a nurse, phlebotomist or medical technologist.
  • The blood sample is tested or processed by a medical technologist.
  • Results are usually reviewed, returned to the person being tested and interpreted by the physician initially ordering the complete blood count.

virus, or other organism) causing it. White blood cells are larger than red blood cells but fewer in number. When a person has a bacterial infection, the number of white cells increases very quickly. The number of white blood cells is sometimes used to pinpoint an infection or to see how the body is reacting to cancer treatment.

Platelets (thrombocytes) are the smallest type of blood cell. They are essential to the process of blood clotting. When bleeding occurs, platelets swell, clump together, and form a sticky plug that helps to stop the bleeding. If the platelet count is too low, uncontrolled bleeding may occur. If the platelet count is too high, there is a chance of a blood clot forming in a blood vessel. Platelets may contribute to the process of hardening of the arteries (atherosclerosis).

There are three red blood cell indices: mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). They are measured by a laboratory instrument machine that calculates their values from other measurements in a complete blood count. The mean corpuscular volume reflects the average size of red blood cells. The mean corpuscular hemoglobin value reflects the quantity of hemoglobin in an average red blood cell. The mean corpuscular hemoglobin concentration reflects the concentration of hemoglobin in an average red blood cell. These numbers are used in diagnosing different types of anemia.

The hemoglobin value reflects the amount of hemoglobin in blood and is a good measure of the ability of a person’s blood stream to carry oxygen throughout the

QUESTIONS TO ASK YOUR DOCTOR

  • Why is a complete blood count needed?
  • What do the results indicate for my health?
  • What treatment options do I have?

body. A hemoglobin molecule comprises much of he volume of red blood cells. It carries oxygen and gives red blood cells their normal color.

The hematocrit value reflects the amount of space (volume) that red blood cells occupy in the blood. The value is given as a percentage of red blood cells in a volume of blood. For example, a hematocrit of 46 means that 46% of the blood’s volume is comprised of red blood cells. Males and females have different normal hematocrit values.

Normal values for the elements of a complete blood count include the following:

  • Red blood cell (erythrocyte) count: 4.2-5.9 million
  • White blood cell (leukocyte) count: 4,300-10,800
  • Platelet (thrombocyte) count: 150,000-400,000
  • Mean corpuscular volume (MCV): 86-98
  • Mean corpuscular hemoglobin (MCH): 27-32
  • Mean corpuscular hemoglobin concentration (MCHC): 32-36%
  • Hemoglobin (Hgb): 13-18 for men and 12-16 for women
  • Hematocrit (HCT): 45–52% for men and 37–48% for women

A complete blood count can be ordered at any time.

Precautions

Precautions are generally not needed for a complete blood count.

At the time of drawing blood, the only precaution needed is to clean the venipuncture site with alcohol.

Side effects

The most common side effects of a complete blood count are minor bleeding (hematoma) or bruising at the site of venipuncture.

KEY TERMS

Hematoma— A collection of blood that has entered a closed space.

Phlebotomist— Health care professional trained to obtain samples of blood.

Interactions

There are no interactions for a complete blood count.

Resources

BOOKS

Fischbach, F. T. and M. B. Dunning. A Manual of Laboratory and Diagnostic Tests. 8th ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

McGhee, M. A Guide to Laboratory Investigations. 5th ed. Oxford, UK: Radcliffe Publishing Ltd, 2008.

Price, C. P. Evidence-Based Laboratory Medicine: Principles. Practice, and Outcomes. 2nd ed. Washington, DC: AACC Press, 2007.

Scott, M.G., A. M. Gronowski, and C. S. Eby. Tietz’s Applied Laboratory Medicine. 2nd ed. New York: Wiley-Liss, 2007.

Springhouse, A. M.. Diagnostic Tests Made Incredibly Easy! 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2008.

PERIODICALS

Amati, L., M. Chiloiro, E. Jirillo, and V. Covelli. “Early pathogenesis of atherosclerosis: the childhood obesity.” Current Pharmaceutical Design 13, no. 36 (2007): 3696–3700.

James, T. R., H. L. Reid, and A. M. Mullings. “Are published standards for haematological indices in pregnancy applicable across populations: an evaluation in healthy pregnant Jamaican women.” BMC Pregnancy and Childbirth 8, no. 1 (2008): 8–19.

Liao, S. C., M. F. Yang, and I. N. Lee. “Transforming laboratory data to improve medical care for patients with chronic kidney disease.” Journal of Nephrology 21, no. 1 (2008): 74–80.

Lippi, G., A. Bassi, G. P. Solero, G. L. Salvagno, and G. C. Guidi. “Prevalence and type of preanalytical errors on inpatient samples referred for complete blood count.” Clinical Laboratory 53, no. 9-12 (2007): 555–556.

ORGANIZATIONS

American Association for Clinical Chemistry. http://www.aacc.org/AACC/.

American Society for Clinical Laboratory Science. http://www.ascls.org/.

American Society of Clinical Pathologists. http://www.ascp.org/.

College of American Pathologists. http://www.cap.org/apps/cap.portal.

OTHER

American Clinical Laboratory Association. “Information about clinical chemistry.” 2008 [cited February 24, 2008]. http://www.clinical-labs.org/.

Clinical Laboratory Management Association. “Information about clinical chemistry.” 2008 [cited February 22, 2008]. http://www.clma.org/.

Lab Tests On Line. “Information about lab tests.” 2008 [cited February 24, 2008]. http://www.labtestsonline.org/.

National Accreditation Agency for Clinical Laboratory Sciences. “Information about laboratory tests.” 2008 [cited February 25, 2008]. http://www.naacls.org/.

L. Fleming Fallon, Jr, MD, DrPH

Computerized axial tomography seeCT scans

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