The anticoagulant effect of heparin + antithrombin III using purified fibrinogen as substrate.

The anticoagulant effect of heparin in test systems composed of purified factors [antithrombin III (AT-III), fibrinogen and thrombin] was studied. As expected, the sensitivity to heparin depended on the concentrations of AT-III and thrombin, whereas the fibrinogen level was less decisive. In addition, qualitative differences proved important. Thus, the sensitivity to heparin was greater with crude bovine thrombin than with highly purified thrombin from the same species. Further, the sensitivity to heparin increased following removal of cold-insoluble material from the fibrinogen preparation. Finally, during storage of purified AT-III at +4 degrees C for more than 4 weeks, the sensitivity to heparin decreased more rapidly than expected from the amidolytic AT-III assays. Smaller amounts of heparin were required to give a prolonged thrombin clotting time than in whole plasma, indicating that components of normal platelet-poor plasma (besides AT-III and fibrinogen) interfere with the anticoagulant effect of heparin. The present test systems may prove suitable for the detection and evaluation of factors of importance for the so-called heparin tolerance.