[Is the activated partial thromboplastin time suitable for monitoring of thrombosis therapy with high-dose standard heparin?].

The activated partial thromboplastin time (aPTT, PTT) is widely used to monitor therapeutic anticoagulation with standard heparin. However, it has been known for some time that PTT reagents obtained from various manufacturers display different sensitivities to heparin--a fact which often is not taken into account. The study deals with the question whether the sensitivity of the PTT to heparin is additionally influenced by the decrease in the vitamin K-dependent coagulation factors such as occurs after starting oral anticoagulation (OAC). In in-vitro studies the reaction of the PTT was observed after addition of different amounts of heparin to normal plasma, to plasma taken after the start of OAC and during stable OAC. The results show that the PTT tends to be more sensitive to heparin as soon as oral anticoagulation is initiated. This phenomenon already occurs at an early stage of anticoagulant intake where only factor VII is markedly reduced but prothrombin concentration is still in an almost normal range and therefore a clinically sufficient effect of OAC is not to be expected. Identical results are obtained with plasma samples of heparin treated patients before and after the start of OAC; in addition, a scattering of the PTTs is obvious. This leads to overestimation of heparin concentrations and a consequent reduction of dosage at an early, still insufficient stage of OAC. In contrast, the thrombin time shows--independently of OAC--a good correlation to heparin concentrations. Therefore the thrombin time is more appropriate to monitor heparin therapy in the phase when oral anticoagulation is started.