MONITORING OF THERAPEUTIC ANTICOAGULATION WITH HIGH-DOSE STANDARD HEPARIN

The activated partial thromboplastin time (aPTT, PTT) is widely used t o monitor therapeutic anticoagulation with standard heparin. However, it has been known for some time that P?T reagents obtained from variou s manufacturers display different sensitivities to heparin - a fact wh ich often is not taken into account. The study deals with the question whether the sensitivity of the PTT to heparin is additionally influen ced by the decrease in the vitamin K-dependent coagulation factors suc h as occurs after starting oral anticoagulation (OAC). In in-vitro stu dies the reaction of the PTT was observed after addition of different amounts of heparin to normal plasma, to plasma taken after the start o f OAC and during stable OAC. The results show that the P?T tends to be more sensitive to heparin as soon as oral anticoagulation is initiate d. This phenomenon already occurs at an early stage of anticoagulant i ntake where only factor VII is markedly reduced but prothrombin concen tration 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 af ter the start of OAC; in addition, a scattering of the PTTs is obvious . This leads to overestimation of heparin concentrations and a consequ ent reduction of dosage at an early, still insufficient stage of OAC. In contrast, the thrombin time shows - independently of OAC - a good c orrelation to heparin concentrations. Therefore the thrombin time is m ore appropriate to monitor heparin therapy in the phase when oral anti coagulation is started.