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SummaryDuring the platelet release reaction β-thromboglobulin (βTG), platelet factor 4 (PF4) and thrombospondin (TSP) are released from the platelet into plasma and assays of these proteins can be used to monitor in vivo platelet activation. We have assessed their relative merits as markers of the in vivo platelet α-granule release reaction in a number of patient groups which have previously been shown to have elevated plasma βTG and/or PF4 levels. It is concluded that in diseases or conditions not complicated by its reduced clearance, βTG is the most sensitive marker of in vivo platelet α-granule release. However, the TSP assay may be the least ambiguous when monitoring the platelet α-granule release reaction in patients with renal failure who are undergoing haemodialysis with heparin anticoagulation. Under these circumstances plasma βTG, but not PF4 or TSP, levels are elevated because of impaired renal catabolism, and the presence of a heparin-releasable reservoir of PF4 on the endothelium complicates the use of the PF4 assay. In liver failure none of these assays may accurately reflect platelet α-granule release because of impaired hepatic or renal elimination of the proteins.

SummaryThe in vivo platelet release reaction in 22 patients with myeloproliferative disorders has been studied by measuring plasma concentrations of the platelet release product β-throm-boglobulin (βTG). Mean βTG and mean βTG:whole blood platelet count ratio were significantly raised in the patient group taken as a whole compared to an age matched control group. No significant increases were observed in the plasma concentrations of thrombin and plasmin sensitive fibrinogen fragments fibrinopeptide A (FpA) and Bβ1-42. The patients were divided into those who had normal, increased or decreased responses to in vitro ADP-induced platelet aggregation. Mean βTG and the mean βTG:whole blood platelet count ratio were higher in the increased and decreased responders to ADP than in the normal aggregation group, but the differences in means were not statistically significant. Aspirin given to six patients at a dose sufficient to eliminate the secondary phase of ADP-induced platelet aggregation reduced mean βTG and the mean βTG : whole blood platelet count ratio but did not alter mean FpA and Bβ1-42. It is concluded that the enhanced platelet release reaction seen in myeloproliferative disorders is independent of plasma protease activity that arises when coagulation and fibrinolytic systems are activated.

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