The Coagulant Activity of Platelets

When platelet‐rich plasma is incubated for 16–20 hours at 37° C. and the platelets are then separated and washed, tissue‐factor activity develops in these platelets. The active platelets accellerate the clotting of plasma samples deficient in Factor XII, XI, IX or VIII, and of normal plasma; the coagulant activity for samples deficient in Factors V, VII and X is much less marked. The activity developed will cause activation of Factor X in a serum eluate, whereas no such activity is evident in platelets separated from plasma immediately after collection from the donor. The development of tissue‐factor activity of platelets depends on the presence of Factor XII but not on the presence of any of the other factors tested. The relationship of tissue‐factor activity to platelet factor 3 is discussed.