RBC indices

Definition

Red blood cell (RBC) indices are part of the complete blood count (CBC) test. They are used to help diagnose the cause of anemia, a condition in which there are too few red blood cells.

The indices include:

• Average red blood cell size (MCV)
• Hemoglobin amount per red blood cell (MCH)
• The amount of hemoglobin relative to the size of the cell (hemoglobin concentration) per red blood cell (MCHC)

See also: RBC count

Alternative Names

Erythrocyte indices; Blood indices; Red cell mass measurement; Mean corpuscular hemoglobin (MCH); Mean corpuscular hemoglobin concentration (MCHC); Mean corpuscular volume (MCV); Red blood cell indices

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The values for MCHC, and MCH are calculated from the hemoglobin (Hgb), hematocrit (Hct), and red blood cell count (RBC):

• MCHC = Hgb/Hct
• MCH = Hgb/RBC count

The MCV is measured directly by a machine.

How to prepare for the test

No special preparation is necessary.

How the test will feel

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

Why the test is performed

RBCs transport hemoglobin which, in turn, transports oxygen. The amount of oxygen tissues receive depends on the amount and function of RBCs and hemoglobin.

The MCV reflects the size of red blood cells. The MCH and MCHC reflect the hemoglobin content of red blood cells. These RBC measures are used to diagnose types of anemia.

Anemias are defined based on cell size (MCV) and amount of Hgb (MCH).

• MCV less than lower limit of normal: microcytic anemia
• MCV within normal range: normocytic anemia
• MCV greater than upper limit of normal: macrocytic anemia
• MCH less than lower limit of normal: hypochromic anemia
• MCH within normal range: normochromic anemia
• MCH greater than upper limit of normal: hyperchromic anemia

Normal Values

• MCV: 80 to 100 femtoliter
• MCH: 27 to 31 picograms/cell
• MCHC: 32 to 36 grams/deciliter

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

This test is used to diagnose the cause of anemia. The following are the types of anemia and their causes:

• Normocytic/normochromic (NC/NC) anemia is caused by sudden blood loss, prosthetic heart valves, sepsis, tumor, long-term disease or aplastic anemia.
• Microcytic/hypochromic anemia is caused by iron deficiency, lead poisoning, or thalassemia.
• Microcytic/normochromic anemia results from a lack (deficiency) of the hormone erythropoietin from kidney failure.
• Macrocytic/normochromic anemia results from chemotherapy, folate deficiency, or vitamin B-12 deficiency.

What the risks are

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

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

References

Zuckerman K. Approach to the anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 162.