Platelets have been separated according to buoyant density using a colloidal silica-polyvinylpyrrolidone system and subjected to electronic sizing. All density populations were found to be heterogeneous in size, the most dense platelets ranging from less than 3 fl to greater than 21 fl in both man and rat. Light platelet fractions contained no platelets greater than 13 fl in either species. Cohort labeling with [75Se]selenomethionine showed no indication of significant change in platelet buoyant density with ageing; greater specific activity found in young, dense platelets appears to be related to increased protein synthetic activity shown in vitro and likely to occur also in their precursor megakaryocytes. It is postulated that dense, intermediate and light platelets are released synchronously by the three different ploidy classes of megakaryocyte, that varying density indicates differing structural characteristics and presumably differences in function. The present findings do not deny the possibility that platelets decrease in size with ageing but if such occurs, it is not associated with a significant change in platelet buoyant density.

A radioimmunoassay has been developed to measure platelet factor 4 (PF4) in biological fluids both in vitro and in vivo. The assay has been shown to be highly specific for PF4 and has a sensitivity of 0.08 ng/assay tube and 1.6 ng/ml of plasma. The preparation of plasma for the measurement of in vivo levels of PF4 requires the use of an anticoagulant containing EDTA, theophylline and prostaglandin E1, the immediate cooling of the blood and high speed or prolonged centrifugation to reduce platelet contamination. Plasma levels of PF4 are normally between 4 and 24 ng/ml with a median of 7.4 ng/ml. Plasma PF4 levels are markedly increased during cardiopulmonary bypass surgery with shortened 51Cr-labelled platelet survival times and during arterial thrombosis. However, despite similarly shortened platelet survival times, the level of PF4 is normal in immune thrombocytopenia. Elevations of plasma PF4 levels are found following surgery, acute myocardial infarction and frequently during acute infections and in inflammatory states. On the contrary, normal levels are usual in disseminated malignancy, in severe hepatic and renal disease and in chronic arterial disease. The measurement of PF4 is a useful addition to the study of platelet pathophysiology. It is apparent, however, that raised levels are by no means specific for thromboembolic disease and similarly platelet destruction is not invariably associated with abnormally increased plasma PF4 levels.