The red blood cell indices (RBC indices) are part of a routine blood test called the complete blood count (CBC). The CBC is used as a measure of general health. It measures the quantity and physical characteristics of different populations of cells found in the blood.
Blood consists of red blood cells, white blood cells, and platelets suspended in plasma. Red blood cells contain hemoglobin, which carries oxygen throughout the body. A red blood cell is pale red and gets its color from hemoglobin. Red blood cells are doughnut-shaped, but with a thinner area in the middle instead of a hole. Your red blood cells are normally all the same color, size, and shape, but certain conditions can cause variations, which impair their ability to function properly.
The RBC indices measure the size, shape, and physical characteristics of the red blood cells. Along with the red blood cell count (RBC), the RBC indices are used to diagnose various forms of anemia. A person with a low RBC and/or any abnormal RBC indices has some form of anemia.
Red blood cells are produced in the bone marrow. They normally live about 120 days. Anemia is a condition in which the number of red blood cells and/or the amount of hemoglobin in the blood falls below normal levels. This deprives tissues throughout the body of needed levels of oxygen.
This can happen if:
- too few red blood cells are created (aplastic anemia)
- red blood cells are destroyed prematurely (hemolytic anemia)
- a significant blood loss occurs
Anemia may be acute or chronic. It may be acquired or inherited. The most common symptoms of anemia are fatigue and lack of energy. As the disease progresses, symptoms may include dizziness, a feeling of cold or numbness in the hands and feet, shortness of breath, irregular or fast heartbeat, and chest pains.
The causes of anemia include:
- diets lacking in iron, vitamin B-12, or folate (folic acid)
- chronic diseases like cancer, diabetes, inflammatory bowel disease, kidney disease, or thyroiditis (inflamed thyroid gland)
- chronic infections like HIV or tuberculosis
- significant blood loss
- diseases affecting the bone marrow, such as leukemia, lymphoma, or multiple myeloma
- certain genetic diseases, such as thalassemia (an inherited form of anemia) or sickle cell disease (in which the red blood cells are sickle-shaped, which affects how well they carry oxygen)
- lead poisoning
Iron deficiency anemia is the most common kind of anemia. The treatment for any anemia depends on its cause(s). The RBC and the RBC indices can help to determine the cause(s) of anemia.
There are three components to the RBC indices:
- the average red blood cell size, called the mean corpuscular volume (MCV)
- the amount of hemoglobin per red blood cell, called the mean corpuscular hemoglobin (MCH)
- the amount of hemoglobin relative to the size of the cell or hemoglobin concentration per red blood cell, called the mean corpuscular hemoglobin concentration (MCHC)
MCV measures the average size of the red blood cells.
The MCV rises when red blood cells are larger than normal (macrocytic). This condition is called macrocytic anemia.
The MCV falls when red blood cells are smaller than normal (microcytic). This condition is called microcytic anemia.
When MCV values are normal, the red blood cells are normal in size. They are called normocytic. However, a person with normocytic red blood cells can still be anemic when there are too few red blood cells or when other RBC indices are abnormal. This is called normocytic anemia.
MCH measures the average amount of hemoglobin in a red blood cell. It is a calculation based on two other values measured in the CBC:
- the hemoglobin value (Hgb), which measures the amount of hemoglobin in the blood
- the RBC, which measures the number of red blood cells in the blood
The MCH is the Hgb divided by the RCB. So MCH = Hgb/RBC.
When MCH values are high, the red blood cells are called hyperchromic. The red blood cells will be deeper in color. This condition is called hyperchromic anemia.
When MCH values are low, the red blood cells are called hypochromic. The red blood cells will be paler in color. This condition is called hypochromic anemia.
When MCH values are normal, the red blood cells are normal in color. They are called normochromic. However, a person with normochromic red blood cells can still be anemic, when there are too few red blood cells or when other RBC indices are abnormal. This is called normochromic anemia.
The terms “hyperchromic” and “hypochromic” are also used to describe high MCHC and low MCHC values, respectively.
The MCHC is the average concentration of hemoglobin inside a red blood cell. The MCHC is calculated using two other values in the CBC:
- the Hgb, which measures the amount of hemoglobin in the blood
- the hemocrit (Hct), which measures the percentage of red blood cells in a given volume of blood
The MCHC is Hgb divided by the Hct. So MCHC = Hgb/Hct.
When the MCHC is higher than normal, then hemoglobin is abnormally concentrated in red blood cells. The red blood cells will be deeper in color. This is called hyperchromia.
When the MCHC is lower than normal, hemoglobin is abnormally diluted in the red blood cells. The red blood cells will be paler in color. This condition is called hypochromic anemia.
When the hemoglobin concentration is normal, the MCHC is normal. A person with normal MCHC values can still be anemic, if there are too few red blood cells or other RBC indices are abnormal. This condition is called normochromic anemia.
According to the National Institutes of Health, normal values for RBC indices are:
- MCV: 80 to 100 femtoliter
- MCH: 27 to 31 picograms/cell
- MCHC: 32 to 36 grams/deciliter (NIH, 2011)
Normal ranges may vary slightly from lab to lab.
If the RBC is low or any of the RBC indices are abnormal, some form of anemia is indicated. If the values are high, it is a condition called polycythemia. It is necessary to consider all these values to get the right diagnosis. Other tests may also be necessary.
Larger red blood cells can hold more hemoglobin. Smaller red blood cells often have less. Thus, the MCH may mirror the MCV.
When the average amount of hemoglobin per red blood cell is decreased, then the concentration is decreased. So, when the MCV is decreased, the MCHC is decreased. Increases in the MCHC are limited by the amount of hemoglobin that can fit inside a red blood cell.
Severe burns or a disorder called hereditary spherocytosis can cause high MCHC values.
Normocytic/normochromic anemia occurs when the red blood cells are normal in size and hemoglobin content, but there are too few of them. This can be caused by sudden and significant blood loss, a prosthetic heart valve, a tumor, a chronic disease, or aplastic anemia.
Macrocytic/normochromic anemia can be caused by B-12 and/or folate deficiency, or chemotherapy.
Microcytic/normochromic anemia can be caused by kidney failure.
Microcytic/hypochromic anemia can be caused by iron deficiency, lead poisoning, or an inherited kind of anemia called thalassemia.
Polycythemia vera typically results from a production of too many red blood cells, and is a disorder of the bone marrow. Sometimes the white blood cells and the platelet counts are also increased. It is a rare disorder and is generally related to a genetic mutation in a gene called JAK2.
No preparation is necessary for this test. A finger stick can be used in both adults and children. A heel stick can be used for infants. The technician will clean the area with an antiseptic. After the test, a bandage can be used to protect the area.
In adults, the blood can also be taken from a vein in the inner elbow. This is called a venipuncture. The technician will clean the test area with an antiseptic and wrap an elastic band around the upper arm to make the vein swell. Then the needle is gently inserted, and blood flows into a tube. When the tube is filled, the technician removes the elastic band and then the needle. A bandage may be placed over the area where the needle was inserted.
Normally, there is some pain when skin is pierced, as well as some throbbing afterward.
It can be helpful to ask for ice to be applied to the area to be cut. This will numb the area to some extent.