A reticulocyte count measures the percentage of reticulocytes (slightly immature red blood cells) in blood.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For infants and small children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

A special stain can be used to identify reticulocytes, because they contain a substance called RNA, while fully mature red blood cells do not. The number of reticulocytes in the blood indicates how quickly they are being produced and released by the bone marrow. Normally, the body will respond to bleeding or hemolytic anemia (breakdown of red blood cells) by an increased rate of red blood cell production. A lower-than-normal percentage of reticulocytes usually indicates a nutritional deficiency.

No special preparation is necessary for adults.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

The test is done to determine if red blood cells are being created in the bone marrow at an appropriate rate.

The normal range depends on the level of hemoglobin, and the range is higher if there has been bleeding or red cell destruction.

A higher-than-normal percentage of reticulocytes may indicate:

• Bleeding
• Erythroblastosis fetalis
• Hemolytic anemia
• Kidney disease with increased erythropoietin production

• Bone marrow failure (for example, from toxicity, tumor, or infection)
• Cirrhosis of the liver
• Folate deficiency
• Iron deficiency
• Kidney disease with decreased erythropoietin production
• Radiation therapy
• Vitamin B-12 deficiency

• Anemia of chronic disease
• Congenital spherocytic anemia
• Drug-induced immune hemolytic anemia
• Hemolytic anemia due to G6PD deficiency
• Idiopathic aplastic anemia
• Idiopathic autoimmune hemolytic anemia
• Immune hemolytic anemia
• Pernicious anemia
• Secondary aplastic anemia

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

The reticulocyte count may be increased during pregnancy.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:491-2.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2675.