Of the three time-honored RBC indices, it is only the mean corpuscular volume, (MCV) index that is alive and well. It is measured today directly by the automated instruments. It permits computation of the hematocrit reading (MCV × number of RBCs) as opposed to the preautomation era when the hematocrit reading was measured and the MCV calculated. The MCV allows a useful morphologic differential diagnosis of microcytic and macrocytic anemias.
In contradistinction, the other two indices, mean corpuscular hemoglobin concentration (MCHC) and mean corpuscular hemoglobin (MCH) are of questionable use. The MCH, also calculated by automated instruments, varies in strict linear relationship with the MCV (Fig 1). It does not, therefore, offer any additional information over the MCV value. However, the MCHC is practically constant (Fig 2) with a mean of 34.8 ± 1.53 (mean ± SD).
A low MCHC, traditionally the characteristic feature of iron deficiency, is not seen