Mean platelet volume. The inverse relation of platelet size and count in normal subjects, and an artifact of other particles.

We measured whole-blood mean platelet volume (MPV) and platelet count, determined by the Coulter Counter model S-Plus, in 683 normal subjects. There was nonlinear, inverse relation between MPV and platelet count throughout the normal range of platelet count: the change in MPV was most pronounced at the lower platelet counts. Because of this non-linear relation, "platelet-crit," the product of platelet count and MPV, was not constant, but varied directly with platelet count. Thus, the definition of "normal" values for MPV requires simultaneous reference to platelet count. In the whole blood of 13 patients, non-platelet cell fragments spuriously increased the automated platelet count. Seven patients with erythrocyte fragmentation had spuriously high MPV also: correct MPV and platelet count could be obtained from platelet rich plasma (PRP). In five patients with leukemic blast fragmentation, MPV remained spuriously low and platelet count spuriously high in PRP: only phase count allowed correct count. A patient with megakaryoblastic leukemia had megakaryoblast fragments confused for platelets.