Blood is drawn from a vein, usually on 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 an infant or young child:

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.

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.

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.

This test can help diagnose anemia and other conditions affecting red blood cells.

The RBC count is almost always part of the CBC (complete blood count) test.

Normal results vary, but in general the range is as follows:

• Male: 4.7 to 6.1 million cells per microliter (cells/mcL)
• Female: 4.2 to 5.4 million cells/mcL

Damaged RBCs do not live as long as normal. Injury inside the blood vessels, such as that caused by artificial heart valves or peripheral blood vessel disease, can damage RBCs.

Higher-than-normal numbers of RBCs may be due to:

• Congenital heart disease
• Cor pulmonale
• Dehydration (such as from severe diarrhea)
• Pulmonary fibrosis
• Polycythemia vera

Lower-than-normal numbers of RBCs may be due to:

• Anemia (various types)
• Bone marrow failure (for example, from radiation, toxin, or tumor)
• Erythropoietin deficiency (secondary to kidney disease )
• Hemolysis (RBC destruction) from transfusion reaction
• Hemorrhage (bleeding)
• Leukemia
• Malnutrition
• Multiple myeloma
• Nutritional deficiencies of:
  • Iron
  • Copper
  • Folate
  • Vitamin B-12
  • Vitamin B-6
• Overhydration

Additional conditions under which the test may be performed:

• Alport syndrome
• Drug-induced immune hemolytic anemia
• Hemolytic anemia due to G6PD deficiency
• Hereditary anemias, such as thalassemia
• Idiopathic autoimmune hemolytic anemia
• Immune hemolytic anemia
• Macroglobulinemia of Waldenstrom
• Paroxysmal nocturnal hemoglobinuria (PNH)
• Primary myelofibrosis
• Renal cell carcinoma

• 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

Pregnancy can cause a decrease in RBCs.

Your RBC count will increase for several weeks when you move to a higher altitude. Dehydration also increases the RBC count.

Drugs can increase the RBC count include:

• Gentamicin
• Methyldopa

Drugs that can decrease the RBC count include:

• Chloramphenicol
• Hydantoins
• Quinidine

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:461-2.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2733.