Reduced platelet factor X-activating activity: a possible contribution to bleeding complications in polycythaemia...

A recently described platelet coagulant activity, factor X-activating activity (FXAA), was studied in 33 patients with polycythaemia vera and in 5 with essential thrombocythaemia, in order to gain information about possible mechanisms responsible for the haemostatic disorders observed in myeloproliferative diseases. Other parameters of platelet function including bleeding time, spontaneous and ADP-, epinephrine- or collagen-induced platelet aggregation, serotonin content and malondialdehyde (MDA) production in response to thrombin, were investigated simultaneously. Compared to the range obtained from 27 control subjects (60-150%), FXAA was low in all 9 patients with bleeding complications (range: 5-39%), in 1 out of 7 patients with thrombotic manifestations (range: 38-200%) and in 9 out of 22 patients without symptoms (range: 38-500%). Only the bleeding group differed significantly from each of the others. Moreover, of all patients with reduced FXAA, those with bleeding could be clearly distinguished from those without such symptoms on the basis of the degree of the defect. Increased FXAA (above 150%) was found in 2 out of 7 thrombotic patients and in 2 out of 22 asymptomatics. MDA production was significantly lower in the haemorrhagic group and enhanced in the thrombotic one as compared to control subjects or asymptomatic patients. However, a large area of overlap was observed between the four groups. None of the other platelet function parameters studied (bleeding time, platelet aggregation, platelet serotonin content) correlated with the clinical type of haemostatic complication. Our results suggest a significant association between reduced FXAA and bleeding tendency. These findings may be of relevance in understanding the pathogenesis of abnormal bleeding in patients with polycythaemia vera and essential thrombocythaemia.