A reticulocyte count is a test that measures the rate at which new (immature) red blood cells (called reticulocytes) are manufactured by bone marrow and released into the bloodstream already containing mature red blood cells. The test is usually performed when medical professionals suspect a patient has anemia or during treatment for anemia, but is also used for other conditions relating to blood disorders.
The reticulocyte count test will determine if immature red blood cells are being produced at an increased rate, a normal rate, or at a decreased rate. The test is also used to monitor and evaluate the response of the bone marrow during treatment for anemia.
On the first day that bone marrow makes a red blood cell, it is a large bluish-colored cell filled with ribonucleic acid (RNA). Over the next two days, the cell diminishes in size and changes to a pinkish color as the amount of RNA decreases and the amount of hemoglobin increases. The cell's nucleus then clusters together. The cell is now considered a reticulocyte, an immature red blood cell. After about three days inside the bone marrow, the reticulocyte is ejected into the bloodstream. The reticulocyte circulates for about two days until it becomes a mature red blood cell—after about a total of five days of maturation. Normally, at any given time, about 1-2% of all red blood cells in the bloodstream are reticulocytes.
The reticulocyte count may be adversely affected by:
- medications such as azathioprine (Imuran), chloramphericol (Chloromycetin), corticotrophin (ACTH), levodopa (L-dopa), and dactinomycin (Cosmegen), along with medications to treat fever, cancer, or malaria
- radiation therapy
- sulfonamide antibiotics (such as Bactrim or Septra)
- recent transfusion of blood
The first step in measuring the reticulocyte count is to perform a blood draw. Blood is drawn by a medical professional using a needle inserted in a vein usually located on the arm or hand. The area is cleaned with antiseptic or alcohol. An elastic band is placed around the upper arm to apply pressure to the vein. The needle is inserted into a vein and approximately 17 ounces (5 mL) of blood is collected in an airtight vial or syringe. After the approximate 5-10 minute procedure, the band is removed to reinstate circulation. After the collection is complete, the needle is removed and the site is covered with a gauze pad or a cotton ball to stop any bleeding. Pressure is applied for a few minutes with the arm slightly elevated. A small bandage is placed over the gauze pad or cotton ball.
The blood sample is then delivered to a laboratory for analysis. A technician places the sample in a test tube with methylene blue, a dye that identifies RNA, a protein found in immature red blood cells but not in mature ones. Drops of the mixture are smeared on slides and viewed under a microscope. The reticulocytes are seen as deep dark blue granules or a blue mesh network. A technician will count one thousand red blood cells manually, determining which ones are reticulocytes and which ones are not, or let a flow cytometer automatically count between (usually) 10,000 and 50,000 red blood cells. In either case, the final reticulocyte count is stated as a percentage of reticulocytes to the total number of red blood cells. The number of reticulocytes in the blood indicates how quickly or slowly they are being produced and released by the bone marrow.
The medical professional should be aware of any prescription medications that the patient is taking before performing the blood draw. The health care professional should observe universal precautions when drawing blood.
The bandage placed over the puncture area should be removed by the patient later in the day and the area cleaned with soap and water.
When the needle is inserted to draw blood, some people feel some pain, while others feel a tiny prick of the skin. Some people feel minor pain or discomfort while the needle is in the vein. Other people feel no pain. The amount of pain is usually determined by the skill of the medical professional, the condition of the patient's veins, and the patient's sensitivity to pain. Afterwards, there may be some minor throbbing of the arm or bruising at the blood draw site.
The main complication is a slight chance of mild dizziness or fainting immediately after the blood draw. Other small risks include excessive bleeding, hematoma, infection, inflammation, multiple punctures, or localized bruises or swelling. People with bleeding disorders may bleed more than usual.
A reticulocyte count is usually given as the percentage of immature red blood cells to the total number of red blood cells in the bloodstream. A normal range, which varies among laboratories, is from 0.5-2.0%. Generally, women and children have higher reticulocyte counts than men.
A lower-than-normal percentage of reticulocytes usually indicates decreased production of red blood cells, which may be caused by a nutritional deficiency. It may also indicate a need for a bone marrow biopsy and further testing, to determine whether there is a problem with the production of new reticulocytes by the bone marrow. Conditions that may indicate a lower-than-normal count include:
- bone marrow failure from such conditions as toxicity, tumor, fibrosis, or infection
- cirrhosis of the liver
- folate (folic acid), iron (Fe), or vitamin B12 deficiencies
- radiation therapy
- kidney disease with decreased erythropoietin production
A higher-than-normal percentage indicates an increased production of red blood cells. This reading shows an indication that there has been blood loss (excess bleeding) or red blood cell breakdown or destruction (hemolysis). It may also indicate a need for further tests. These conditions may be present:
- erythroblastosis fetalis
- h anemia
- post-hemorrhage (bleeding)
- kidney disease with increased erythropoietin production
Health care team roles
The medical team is responsible for administering the blood draw, analyzing the blood sample, providing results to the patient, and determining a course of action based on the results of the reticulocyte count.
Anemia— Condition characterized by a decreased number and/or size of red blood cells.
Erythroblastosis fetalis— Condition in which pregnant women make antibodies that attack the fetuses' red blood cells.
Erythropoietin— Glycoprotein hormone that stimulates bone marrow production of red blood cells.
Hematoma— Blood pooling under the skin; a bruise.
Hemoglobin— Oxygen-carrying protein in red blood cells.
Hemolytic anemia— Breakdown of red blood cells.
Post-hemorrhage— Excessive bleeding.
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