Effects of unfractionated and fractionated heparin on platelet function.

Heparin-induced alterations of platelet function have repeatedly been reported over the last 30 years. The development of low-molecular-weight (LMW) heparin fractions prompted us to compare the effect of unfractionated (UF) heparin and LMW heparin on platelet function. Heparin was applied intravenously to healthy volunteers in a dose of 100 U (antifactor Xa)/kg body weight. Sequential evaluation of platelet function ex vivo confirmed that UF heparin may activate platelets in vivo. Studying the ADP-induced fibrinogen binding to platelets in vivo, we were able to show that the effect of heparin on platelets is antibody- and antithrombin-III-independent. LMW heparin fractions in general exhibit a reduced platelet activating activity although there are differences between the LMW heparin fractions available. The stimulating effect of LMW heparin on platelets disappears below a mean molecular weight of 3,000 daltons.