Nonequivalence of automated and manual hematocrit and erythrocytic indices

Manual and automated (Coulter) methods for determining hematocrit and mean corpuscular volume (MCV) were compared using blood specimens in which normal erythrocytes had been serially diluted in their own plasma. Thus, predicted linearity of hematocrit and predicted constancy of the erythrocytic indices were evaluated by these technics. Automated hematocrit was a rectilinear function of the erythrocyte count. Centrifuged hematocrit deviated systematically from the automated hematocrit; its correlation with the erythrocyte count was curvilinear. Automated erythrocytic indices were nearly constant throughout the range of dilutions of erythrocytes. The MCV, calculated from centrifuged hemtocrit and automated erythrocyte count, varied strikingly as a function of the latter. The range of MCV so observed was 62 to 105 fl for normal blood specimens, serially diluted, which, when undiluted, had MCV values in the range of 82 to 90 fl. Results of manual (centrifuged) hematocrit and erythrocytic indices appear to be inherently less reliable than those obtained by automated methods.