Platelet parameters and aggregation in essential and reactive thrombocytosis.

Platelet characteristics were assessed in 15 patients with essential thrombocytosis (ET), 89 patients with reactive thrombocytosis (RT), and 23 normal controls. A platelet volume distribution width (PDW) greater than or equal to 10.5 was found in 50%, 21%, and 14% of the three groups, respectively (P = 0.01 between patients with ET and patients with RT; P = 0.02 between patients with RT and controls), reflecting an excess of extreme values at both ends of the distribution. Compared with controls, the increase in platelet number in patients with RT was about twofold throughout the platelet volume range, whereas ET was characterized by a fivefold increase in small platelets less than 7.5 fL and threefold increase in larger size platelets. Mean platelet volume (MPV) was significantly lower in patients with ET versus patients with RT and in patients with RT versus controls (mean +/- SD 7.5 +/- 1.2 vs. 8.8 +/- 0.1 and 10.2 +/- 1.8 fL, respectively, P less than 0.01). Rate of in vitro platelet aggregation greater than or equal to 50% was significantly lower in patients with ET versus patients with RT and in patients with RT versus controls (0%, 23%, and 45%, respectively, P less than 0.01). Aggregation rate was positively correlated with MPV (r = 0.54; P less than 0.0001). Aggregation rate in patients with ET was significantly lower (P = 0.01) than expected from their reduced MPV alone. Despite these group differences, the overlap of individual platelet characteristics between the three groups precludes their usefulness for diagnostic purposes.