Electronic platelet counts with the Coulter counter. Reassessment of a correction factor.

Platelet counts are determined on the Coulter electronic counter by counting the diluted platelet-rich plasma obtained by sedimentation or centrifugation of whole blood. In calculating the whole-blood platelet count, an empirical correction factor for platelet-free plasma trapped by sedimented erythrocytes has been recommended, and a widely-distributed circular slide rule calculator incorporates the correction factor. In this study, visual and electronic platelet counts were compared in 100 specimens with counts ranging from 10 to 1,100 X 10(3) per mul and hematocrits ranging from 17.5 to 48.5%. Platelet-rich plasma samples prepared by a centrifugation method (Plateletfuge) gave machine counts in close agreement with those of samples prepared by sedimentation. Whole-blood platelet counts determined with the circular calculator were consistently lower than visual counts, with an average difference of -17%. The electronic counts were recalculated after elimination of the correction factor, and agreement then improved to an average difference of only +1.6%. The correction factor for trapped platelet-free plasma leads to erroneously low values and should not be used.