Ratios of anti-factor Xa to antithrombin activities of heparins as determined in recalcified human plasma

unfractionated (UF) and low molecular weight (tMW) heparins have been measured in human plasma and with purified human antithrombin III (ATIII) in the absence and presence of 1'5 mu calcium. The anti-factor Xa and anti-thrombin activities were measured directly, by assessing the heparin-dependent pseudo-first order rate constants of inacti-vation of human factor Xa or thrombin. These activities were studied with the 4th International Standard for UF heparin, the lst International Standard for LMW heparin, C'1216, enoxaparin, Cy222, and the synthetic pentasaccharide ' In plasma, calcium equally well increased the specific anti-factor Xa catalytic activities as compared to purified ATIII' That is, 1 '5 mu calcium stimulated the UF standard heparin-catalysed inactivation of factor Xa 2'L-2'4 times ' In the presence of the LMW heparins the effect of calcium wa