Contribution of platelet protein S (PS) to PS levels measured in plasma

Protein S (PS) is a plasma protein synthesized by hepatocytes and endothelial cells and also found in platelets. Since it has previously been suggested that hereditary PS deficiency may sometimes remain undetected due to enhanced release of platelet PS into plasma, we investigated this question by measuring and comparing PS levels in plasma obtained using procedures susceptible to induce different degrees of platelet activation or disruption. Immunological and functional assays were employed. Plasma samples were obtained from blood centrifuged immediately on collection or after 3 h storage at room temperature, or following repeated freeze-thawing of platelet rich plasma. Analysis of variance revealed no significant difference in PS levels among the samples prepared by these different procedures. It is therefore doubtful that release of platelet PS content could mask a PS deficiency in plasma.