SIZE DISTRIBUTION OF ERYTHROCYTES

The measurement of red cell size is beset with difficulties because of the nature of the cell. The red cell-a biconcave disc-is easily distorted when spread on a slide and fixed for microscopic measurements of either diameter or thickness. When measurements are made in suspensions, red cells may undergo volume changes even in media considered physiologic. For these reasons, the most reliably measured parameter of average red cell size is the mean corpuscular volume (MCV) measured indirectly as the ratio of red cell number (determined by an electronic counter) to the packed red cell volume (determined by a microhematocrit method). Both determinations can be performed with an error of less than 2 per cent.' Changes in cell volume or shape do not affect the electronic counts, and the only assumption made is that centrifugation of cells in their own plasma does not change their volume, although shape changes undoubtedly occur. Knowledge of average cell volumes is of considerable value in practical hematology for the classification and treatment of anemias. In many cases, however, further insight into a disease process is gained from the distribution of cell sizes, as demonstrated by PriceJones?#* who measured the distribution of red cell diameters in fixed smears. Unfortunately, this method and its numerous modifications not only involve some distortion of the red cells but also are very laborious. The development of the Model B Coulter Counter, which graphs the size distribution of a red cell sample within 100 sec., thus offers both surcease from tedium and the advantage of sampling an undistorted population. The instrument, therefore, was subjected to a series of tests that form the basis for this report.