Hirudin determination in plasma can be strongly influenced by the prothrombin level

Recombinant hirudin is increasingly used for therapeutic and prophylactic a nticoagulation. Several laboratory methods are available to measure r-hirud in, including clot-based and amidolytic methods. The snake venom ecarin con verts prothrombin to meizothrombin. Hirudin inhibits meizothrombin, causing a prolongation of the ecarin clotting time (ECT). Because the ECT depends on prothrombin levels in plasma, it was compared with a chromogenic substra te assay (CSA) for the determination of r-hirudin levels in prothrombin def icient plasma samples. R-hirudin (0.0-2.0 mug/mL) was added to plasma sampl es with decreasing prothrombin concentrations (100-0%). Using the ECT, fals e high r-hirudin levels were observed even in r-hirudin-free plasma, when p rothrombin levels were below 50%. This effect was more pronounced with incr easing r-hirudin levels. Additionally, r-hirudin (0.5 mug/mL) was added to plasma of patients with acquired prothrombin deficiency due to oral anticoa gulation (n=33). Hirudin levels were also over estimated in these plasma sa mples using ECT. In plasma samples of patients (n=12) treated with r-hirudi n, because of suspected heparin-induced thrombocytopenia (HIT), hirudin lev els were already measured falsely high, when the prothrombin levels were be low 70%. The chromogenic substrate assay (CSA) determined correct values in all prothrombin-deficient plasma samples. Therefore, the CSA should be use d for hirudin level determination, if overestimation due to prothrombin def iciency should be avoided. (C) 2000 Elsevier Science Ltd. All rights reserv ed.