A standardized bioassay for platelet factor 3 released by kaolin.

The test for platelet factor 3 described by Hardisty & Hutton (1975) has been modified to conform to the usual design for a parallel-line bioassay. Manchester Comparative Thromboplastin has been used as assay Standard, allowing an arbitrary unit of activity to be adopted. However, experiments suggested that the platelet activity measured was different from tissue factor activity. Platelets are tested as platelet-rich plasma diluted in a standardized mixture of plasma and fibrinogen, so that differences between the clotting factors of the test samples are eliminated, as verified by experiments on haemophiliacs and patients on anticoagulant treatment. Sonication and repeated freezing and thawing of platelet-rich plasma showed that approximately 15% of the PF3 is released by kaolin. In vivo, a single dose of 600 mg of aspirin reduced the PR3-release to half the previous value in 2 h; initial values were regained in 5-8 days.