Platelet function in patients with high platelet counts.

Platelet function tests were done to ascertain whether they might have discriminatory significance in the differentiation of a myeloproliferative disorder from secondary thrombocytosis in a patient with an elevated platelet count. Amongst 14 subjects with polycythaemia vera and essential thrombocythaemia, significant abnormalities of platelet aggregation were the rule, while defective aggregation was unusual in 16 subjects with secondary thrombocytosis. Measurements of the bleeding time and of platelet factor 3 activity were of some value, but significant overlap was noted. Platelet function in four of five subjects with chronic myeloid leukaemia was essentially normal.