Estimation of platelet counts on feline blood smears

Blood samples were obtained from 50 cats admitted for hematologic evaluation at the Royal (Dick) School of Veterinary Studies. Manual platelet counts were done using a hemacytometer, and the average number of platelets per oil immersion field (1,000X magnification) was determined on stained blood smears. A hemacytometer count was not obtained for one sample because of a failure in erythrocyte lysing. In nine samples, obvious platelet clumps in the blood smear prevented accurate determination of the number of platelets per oil immersion field. Hemacytometer counts on these nine samples ranged from 260-587 X 10(3) platelets/mu L, suggesting that platelet clumps on a blood smear were usually associated with adequate platelet numbers. Simple regression analysis of hemacytometer counts and the average number of platelets per oil immersion field for the remaining 40 samples yielded correlation coefficients (r) of 0.776 on untransformed data, and 0.892 on log(10)-transformed data. Each platelet per oil immersion field represented a circulating platelet count of similar to 20 X 10(3)/mu L, similar to conversion factors reported for dogs and human beings. It was concluded that estimation of platelet number on stained blood smears is a simple and quick method that appears to be reliable over a wide range of platelet counts in cats.