Estimation of haemoglobin concentration, erythrocyte count, haematocrit, total and differential leucocyte count and platelet count along with examination of the peripheral smear are a part of evaluation of anaemia. Examination of the peripheral smear detects abnormalities in size (microcytosis or microcytosis, anisocytosis), shape and haemoglobnization. Reporting a peripheral smear need experience and is subjective. Red cell indices evolved to bring objectivity and increase reproducibility reporting of red cell characteristics. Red cell indices have allowed a reproducible algorithmic approach to the diagnosis of anaemia.
Normal red cell counts were described in 1852 . This measurement made on four subjects was the normal standard for the next 70 years. Wintrobe in the 1930s determined the normal red cell count described the method for calculating red cell indices (Wintrobe MM Am J Med Sci 1929;177:513–23) and classified anaemia into four groups, macrocytic, normocytic, microcytic and microcytic hypochromic (Wintrobe MM Arch Intern Med. 1934. 54:256-280). Though the methods of determining red cell indices has changed, the concept of classification of red cells by size and the classes of anaemia described by Wintrobe still hold. The three indices are
- Mean corpuscular volume (MCV)
- Mean corpuscular haemoglobin (MCH)
- Mean corpuscular haemoglobin concentration (MCHC)
Mean Corpuscular Volume (MCV)
MCV is the average volume of erythrocytes and is defined as follows:
MCV = [Haematocrit (in percentage) X 10]/RBC count (in millions/mm3)
The normal value is 92±9fl
At the time MCV was described it was calculated as described above. Erythrocyte counts performed manually were cumbersome, technician dependent and inaccurate limiting the value of MCV. Automated haematology counters may be based on impedance or light scatter. Sensors of counters, impedance as well as those based on light scatter, generate signals according to cell size and number. The size of each particle is calculated by the pulse it generates and the MCV reported as an average of these measurements. The standard deviation of observation of MCV is reported as red cell distribution width. When automated counters are used MCV is a measured parameter rather than a calculated parameter. These measurements are accurate and reproducible.
Mean Corpuscular Haemoglobin (MCH) and Mean Corpuscular Haemoglobin Concentration (MCHC)
Mean corpuscular haemoglobin is defined as follows.
MCV = [Haemoglobin (in g/dL) X 10]/RBC count (in millions/mm3)
The normal value for MCH is 29.5±2.5pg.
Mean corpuscular haemoglobin concentration is calculated as follows
MCHC = [Haemoglobin (in g/dL) X100]/[Haematocrit (in percentage)]
The MCV and MCHC are of limited value in diagnosis of anaemias. A low MCH has the same significance as a low MCV. MCHC is low in microcytic anaemias. Macrocytic anaemias may have a high MCH but the MCHC is normal in these case. Some anaemias like hereditary spherocytosis, sickle cell anaemia and haemoglobin C disease are characterized by a high MCHC indicating cellular dehydration