Background: The contact phase of coagulation is of physiologic/pathophysiologic importance, whenever unphysiologic polynegative substances such as cell fragments (microparticles) get in contact with blood. There are several clinically used inhibitors of intrinsic thrombin generation. Here the inhibitory concentrations 50% (IC50) of these anticoagulants are measured by the highly specifi c thrombin generation assay INCA.

Methods: Unfrozen pooled normal citrated plasma in polystyrole tubes was supplemented at 23°C in duplicate with 0–2 IU/ml low molecular weight heparin (dalteparin), 0–2 IU/ml unfractionated heparin, 0–500 KIU/ml aprotinin, or 0–40 mM arginine. 50 μl plasma or 1 IU/ml thrombin standard were pipetted into a polystyrole microtiter plate with fl at bottom. 5 μl SiO2/ CaCl2 - reagent (INCA activator) were added and after 0–30 min incubation at 37°C 100 μl 2.5 M arginine, pH 8.6, were added; arginine inhibits hemostasis activation and depolymerizes generated fi brin within 20 min at 23°C. The in the physiologic 37°C incubation phase generated thrombin was then chromogenically detected. The intra-assay CV values were < 5%.

Results and Discussion: The approximate IC50 were 0.01 IU/ml dalteparin, 0.02 IU/ml heparin, 25 KIU/ml aprotinin, and 12 mM arginine. The effi ciency of any anticoagulant on intrinsic thrombin generation should be measured for each individual patient.

Abbreviations: IIa, thrombin; ΔA, increase in absorbance; APTT, activated partial thromboplastin time; CRT, coagulation reaction time (at 37°C in water-bath); F-wells, polystyrole microtiter plates with fl at bottom; IC50, inhibitory concentration 50%; INCA, intrinsic coagulation activity assay; IU, international units; KIU, kallikrein inhibiting unis; LMWH, low molecular weight heparin; mA, milli-absorbance units; PSL, pathromtin SL®; RT, room temperature (23°C); U-wells, polystyrole microtiter plates with round bottom.