HEMATOLOGY EBOOK

Red blood cell indices are blood tests that provide information about the hemoglobin content and size of red blood cells. Abnormal values indicate the presence of anemia and which type of anemia it is.

Mean corpuscular volume

The mean corpuscular volume, or “mean cell volume” (MCV), is a measure of the average red blood cell volume (i.e. size) that is reported as part of a standard complete blood count. In patients with anemia, it is the MCV measurement that allows classification as either a microcytic anemia (MCV below normal range) or macrocytic anemia (MCV above normal range).

Calculation

It can be calculated (in litres) by dividing the hematocrit by the red blood cell count (number of red blood cells per litre). The results is typically reported in femtolitres. If the MCV was determined by automated equipment, the result can be compared to RBC morphology on a peripheral blood smear. Any deviation would be indicative of either faulty equipment or technician error. For further specification, it can be used to calculate red blood cell distribution width (RDW).

Interpretation

The normal referance range is typically 80-100 fL.

High

In presence of hemolytic anaemia, presence of reticulocytes can increase MCV. In pernicious anemia (macrocytic), MCV can range up to 150 femtolitres. An elevated MCV is also asociated with alcoholism (as are an elevated GGT and a ratio of AST:ALT of 2:1). Vitamin b12 and/or Folic Acid deficiency has also been ssociated with macrocytic anemia (high MCV numbers).

Low

The most common causes of microcytic anemia are iron deficiency (due to inadequate dietary intake, gastrointestinal blood loss, or menstrual blood loss), thalassemia, or chronic disease. A low MCV number in a patient with a positivve stool guaiac test (bloody stool) is highly suggestive of GI cancer. In iron deficiency anemia (microcytic anemia), it can be as low as 60 to 70 femtolitres. In cases of thalassemia, the MCV may be low even though the patient is not iron deficient.

Mean corpuscular hemoglobin

The mean corpuscular hemoglobin, or “mean cell hemoglobin” (MCH), is the average of hemoglobin per red blood cell in a sample of blood. It is reported as part of a standard complete blood count: MCH value is diminished in hypochromic anemias. It is calculated by dividing the total mass of hemoglobin by the number of red blood cells in a volume of blood.

MCH=(Hgb*10)/RBC

A normal value in humans is 27 to 31 picograms/ cell. Conversion to SI-units: 1 pg of hemoglobin = 0,06207 femtomol. Normal value converted to SI-Units: 1,68 – 1,92 fmol/cell.

Mean corpuscular hemoglobin concentration

The mean corpuscular hemoglobin concentration, or MCHC, is a measure of the concentration of hemoglobin in a given volume of packed red blood cell. It is reported as part of a standard complete blood count. It is calculated by dividing the hemoglobin by the hematocrit. Reference ranges for blood tests are 32 to 36 g/dl, or between 4.9 to 5.5 mmol/L. It is thus a mass or molar concentration. Still, many instances measure MCHC in percentage (%), as if it was a mass fraction (m_Hb / m_RBC). Numerically, however, the MCHC in g/dl and the mass fraction of hemoglobin in red blood cells in % are identical, assuming a RBC density of 1g/mL and negligible hemoglobin in plasma.

Interpretation

It is diminished (“hypochromic”) in microcytic anemias, and normal (“normochromic”) in macrocytic anemias (due to larger cell size, though the hemoglobin amount or MCH is high, the concentration remains normal). MCHC is elevated in hereditary spherocytosis.

This count is used to give a rough guide to what shade of red, RBC will be. (paler=lower than the standard)

Complications with cold agglutinin

Because of the way automated analysers count blood cells, a very high MCHC (greater than about 370 g/L) may indicate the blood is from someone with a cold agglutinin. This means that when their blood gets colder than 37°C it starts to clump together. As a result, the analyzer may incorrectly report a low number of very dense red blood cells for blood samples in which agglutination has occurred.

This problem is usually picked up by the laboratory before the result is reported. The blood is warmed until the cells separate from each other, and quickly put through the machine while still warm.This is the most sensitive test for iron deficiency anemia.

Worked example