This is a blood test that measures the number of red blood cells (RBCs). The RBC count is almost always part of the CBC (complete blood count) test.

RBCs transport hemoglobin. Hemoglobin transports oxygen. The amount of oxygen body tissues receive depends on the amount and function of RBCs and hemoglobin. RBCs normally survive about 120 days in the blood. They are then removed by specialized "clean-up" cells in the spleen and liver.

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 circulation. 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.

For infants and children:

The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

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.

This test can help identify anemia (low hemoglobin) as well as other conditions affecting the red blood cells.

RBC (varies with altitude):

• male: 4.7 to 6.1 million cells/mcl
• female: 4.2 to 5.4 million cells/mcl

Abnormal RBCs have a shorter-than-normal life span. Trauma within the blood vessels, such as that caused by artificial heart valves or peripheral vascular (blood vessel) disease, can damage RBCs. An enlarged spleen, as can be caused by portal hypertension or leukemia, may remove normal red cells along with aging ones.

Higher-than-normal numbers of RBCs may indicate:

• congenital heart disease
• cor pulmonale
• pulmonary fibrosis
• polycythemia vera
• dehydration (such as from severe diarrhea)
• kidney disease with high erythropoietin production

Lower-than-normal numbers of RBCs may indicate:

• anemia (various types)
• hemorrhage (bleeding)
• bone marrow failure (for example, from radiation, toxin, fibrosis, tumor)
• erythropoietin deficiency (secondary to kidney disease)
• hemolysis (RBC destruction) from transfusion reaction
• leukemia
• multiple myeloma
• malnutrition

nutritional deficiencies of:

• iron
• folate
• vitamin B-12
• vitamin B-6
• overhydration

Additional conditions under which the test may be performed:

• Alport syndrome
• hemolytic anemia due to G6PD deficiency
• idiopathic autoimmune hemolytic anemia
• immune hemolytic anemia
• macroglobulinemia of Waldenstrom
• paroxysmal nocturnal hemoglobinuria (PNH)
• primary myelofibrosis
• renal cell carcinoma
• drug-induced immune hemolytic anemia
• congenital anemias, such as thalassemia

• 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 RBC count may be decreased during pregnancy because of an increase in body fluids.
• The RBC will increase over a period of several weeks after moving to a higher altitude.
• Dehydration increases the RBC count.
• Drugs that can increase the RBC count include gentamicin and methyldopa. Drugs that can decrease the RBC count include chloramphenicol, hydantoins, and quinidine.
• 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.