Coulter Counter studies of hypotonic-induced macrothrombocytosis in normal subjects and in idiopathic thrombocytopenic purpura patients.

The addition of distilled water to platelet rich plasma converts the platelets into large spherocytes without lysis. This phenomenon implies an evagination of the platelet membrane which participates in the surface connecting system. An adaptation of the direct microscopic method (Milton & Frojmovic, 1979) to the Coulter Counter is reported. The maximum increase in platelet volume without platelet lysis was obtained for 120 mOsm. These volume changes are a consequence of a shift in the platelet volume distribution curve towards higher values without loss of log normality, while the volume dispersion is only slightly modified. This suggests that the amount of evaginable platelet membrane is correlated to the size of the platelet. The size parameters (volume, surface, diameter) of hypotonic-induced spherocytes closely agree with those reported in the original microscopic method. Application of this technique to ITP-platelets suggests a reduced evaginable platelet membrane; this fact probably reflects the abnormal demarcation of platelet territories at the megakaryocytic level.