A new global test for the evaluation of the activated factor II-antithrombin system

We developed a new simple test to evaluate the global function of the activated factor II-antithrombin system. The new test measures the clotting time of plasma samples after the addition of a reagent containing a snake venom (Echis carinatus) that can activate prothrombin, with (Ta) and without (To) heparin. The prolongation of clotting times (Ta - To) is directly related to the function of the activated factor II-antithrombin system. The presence of quantitative or functional defects of the natural inhibitors (antithrombin and heparin co-factor II), or high levels of factor II and/or fibrinogen, can trigger a resistance to the inhibition of activated factor II. This new test was used to examine 134 thrombophilic patients as well as 157 normal subjects as controls. The results obtained confirm that the presence of abnormalities relating to the activated factor II-antithrombin system causes a resistance to activated factor II inhibition even if a significant number of patients was found to have a resistance that could not be accounted for. Since the new test can be easily performed automatically and has a good inter- and intra-assay variation coefficient (CV < 4%) it is useful for evaluating the global function of the activated factor II-antithrombin system in screening thrombophilic patients, alongside the tests already known and used to diagnose these patients.