White blood cell count and differential

Definition

A white blood cell (WBC) count determines the concentration of white blood cells in the patient's blood. A differential determines the percentage of each of the five types of mature white blood cells.

Purpose

This test is included in general health examinations and to help investigate a variety of illnesses. 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. Abnormal counts which return to normal indicate that the condition is improving, while counts that become more abnormal indicate that the condition is worsening. 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.

Precautions

Many medications affect the WBC count. Both prescription and non-prescription drugs including herbal supplements should be noted. Normal values for both the WBC count and differential are age-related.

Sources of error in manual WBC counting are due largely to variance in the dilution of the sample and the distribution of cells in the chamber, as well as the small number of WBCs that are counted. For electronic WBC counts and differentials, interference may be caused by small fibrin clots, nucleated red blood cells (RBCs), platelet clumping, and unlysed RBCs. Immature WBCs and nucleated RBCs may cause interference with the automated differential count. Automated cell counters may not be acceptable for counting WBCs in other body fluids, especially when the number of WBCs is less than 1000/μL or when other nucleated cell types are present.

Description

White cell counts are usually performed using an automated instrument, but may be done manually using a microscope and a counting chamber, especially when counts are very low, or if the patient has a condition known to interfere with an automated WBC count.

An automated differential may be performed by an electronic cell counter or by an image analysis instrument. 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. The WBC differential is performed manually by microscopic examination of a blood sample that is spread in a thin film on a glass slide. White blood cells are identified by their size, shape, and texture.

The manual WBC differential involves a thorough evaluation of a stained blood film. In addition to determining the percentage of each mature white blood cell, the following tests are preformed 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 or immature RBCs; and counting the number of nucleated RBCs per 100 WBCs.
• An estimate of the WBC count is made and compared with the automated or chamber WBC count. An estimate of the platelet count is made and compared with the automated or chamber platelet count. Abnormal platelets, such as clumped platelets or excessively large platelets, are noted on the report.
• Any immature WBCs are included in the differential count of 100 cells, and any inclusions or abnormalities of the WBCs are reported.

Preparation

This test requires a 3.5 mL sample of blood. Vein puncture with a needle is usually performed by a nurse or phlebotomist, a person trained to draw blood. There is no restriction on diet or physical activity.

Aftercare

Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be allowed to lie down and relax until they are stable.

Risks

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

Normal results

Normal values vary with age. White blood cell counts are highest in children under one year of age and then decrease somewhat until adulthood. The increase is largely in the lymphocyte population. Adult normal values are shown below.

• WBC count: 4,500–11,000/μL
• polymorphonuclear neutrophils: 1800–7800/μL; (50–70%)
• band neutrophils: 0–700/μL; (0–10%)
• lymphocytes: 1000–4800/μL; (15–45%)
• monocytes: 0–800/μL; (0–10%)
• eosinophils: 0–450/μL; (0–6%)
• basophils: 0–200/μL; (0–2%)

Resources

books

chernecky, cynthia c., and barbara j. berger. laboratory tests and diagnostic procedures, 3rd ed. philadelphia, pa: w. b. saunders company, 2001.

henry, j. b. clinical diagnosis and management by laboratory methods, 20th ed. philadelphia, pa: w.b. saunders company, 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 & wilkens, 2000.

other

national institutes of health. [cited april 5, 2003] <http://www.nlm.nih.gov/medlineplus/encyclopedia.html>.

Victoria E. DeMoranville Mark A. Best

White Blood Cell Count and Differential

Definition

The white blood cell count and differential determine the number of white blood cells and the percentage of each type of white blood cell in a person's blood. These tests are included in general health examinations and help investigate a variety of illnesses, including infection, allergy, and leukemia.

Purpose

The white blood cell count provides a clue to the presence of illness. White cells protect the body by fighting infection and attacking foreign material. When extra white cells are needed, the bone marrow increases production.

There are five types of white cells, each with different functions: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The differential reveals if these cells are present in a normal distribution, or if one cell type is increased or decreased. This information helps diagnose specific types of illness.

Conditions or medications that weaken the immune system, such as AIDS or chemotherapy, cause a decrease in white cells. The white cell count detects dangerously low numbers of white cells.

Recovery from illness can be monitored by the white cell count. Counts continuing to rise or fall to abnormal levels indicate a worsening condition; counts returning to normal indicate improvement.

Description

Neutrophils increase in response to bacterial infection. They destroy bacteria by enveloping and digesting them, a process called phagocytosis. When many neutrophils are needed, they are released from the bone marrow as immature cells, called bands or stab cells.

Lymphocytes fight viral infections and some bacterial infections. Certain lymphocytes directly attack invading microorganisms; others produce antibodies that attack and destroy microorganisms and other foreign material. Large lymphocytes, called atypical lymphocytes, are seen during infectious mononucleosis and other illnesses.

Monocytes increase during severe infections, and other conditions. They remove debris and microorganisms by phagocytosis. Eosinophils and basophils increase in response to allergic reactions and parasitic infection.

White cell counts are usually done on an automated instrument. A sample of blood is mixed with a chemical to burst the red blood cells. The remaining white cells are counted by the instrument.

The differential is done by spreading a drop of blood on a microscope slide. The slide is stained with a special stain and examined under a microscope. One-hundred white cells are counted and identified as either neutrophils, bands, lymphocytes, monocytes, eosinophils or basophils. Any atypical or immature cells also are counted. Cells are identified by the shape and appearance of the nucleus, the color of cytoplasm (the background of the cell), and the presence and color of granules. The percentage of each cell type is reported. At the same time, red cells and platelets are examined for abnormalities in appearance. Some instruments perform an automated differential.

Both the white blood cell count (also called white count or leukocyte count) and the differential (also called diff) are covered by insurance. Results are available the same day.

Preparation

This test requires 7 mL of blood. A healthcare worker ties a tourniquet on the person's upper arm, locates a vein in the inner elbow region, and inserts a needle into that vein. Vacuum action draws the blood through the needle into an attached tube. Collection of the sample takes only a few minutes.

Aftercare

Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. The person may feel dizzy or faint.

Normal results

Total white cell count 5,000-10,000 uL. Neutrophils 50-60%. Lymphocytes 20-40%. Monocytes 2-6%. Eosinophils 1-4%. Basophils 0.5-1%. Bands 0-3%.

Abnormal results

The white cell count and differential are interpreted according to a person's clinical condition and medical history. Leukocytosis (a white count increased to over 10,000/uL) is seen in bacterial infections, inflammation, leukemia, trauma, and stress. Leukopenia (a white count decreased to less than 4,000/μL) is seen in some viral infections or severe bacterial infections, and conditions that affect the bone marrow such as dietary deficiencies, chemotherapy, radiation therapy, and autoimmune diseases.

KEY TERMS

Band— Immature neutrophil.

Basophil— White blood cell that increases in response to parasitic infections and allergic reactions.

Differential— Blood test that determines the percentage of each type of white blood cell in a person's blood.

Eosinophil— White blood cell that increases in response to parasitic infections and allergic reactions.

Leukocytosis— A white count increased to over 10,000/μL.

Leukopenia— A white count decreased to less than 4,000/μL.

Lymphocyte— White blood cell that fights viral and some bacterial infections by direct attack or the production of antibodies.

Monocyte— White blood cell that increases during a variety of conditions including severe infections. It removes debris and microorganisms by phagocytosis.

Neutrophil— White blood cell that increases in response to bacterial infection. It removes and kills bacteria through phagocytosis.

Phagocytosis— A process by which a white blood cell envelopes and digests debris and microorganisms to remove them from the blood.