ADAM Health Encyclopedia

RBC indices

Definition

Red blood cell (RBC) indices are part of the complete blood count (CBC) test. They are used to help diagnose too few red blood cells (anemia).

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

Why the Test is Performed

Red blood cells 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

How the Test is Performed

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. 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 swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight 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. 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 small glass tube (pipette), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if bleeding continues.

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

The MCV is measured directly.

How to Prepare for the Test

No special preparation is necessary.

How you prepare infants and children for this test depends on your child's age and past experience. For general 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)
• School age test or procedure preparation (6 to 12 years)
• Adolescent test or procedure preparation (12 to 18 years)

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.

Risks

• Excess bleeding
• Fainting or feeling lightheaded
• Blood pooling under the skin (hematoma)
• Infection
• Many needle sticks to find veins

Considerations

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

Normal Results

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

What Abnormal Results Mean

The following are the types of anemia and their causes:

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