LABORATORY ASSESSMENT OF ANTITHROMBOTIC THERAPY - WHAT TESTS AND IF SO WHY

A critical review is given of the tests available for the assessment o f the action of anticoagulants, such as heparins, oral anticoagulants and direct thrombin inhibitors, in patients under antithrombotic thera py. The principle of action and the performance of the thromboplastin time (PT), the activated partial thromboplastin time (aPTT), the whole blood clotting time, the thrombin time, the ecarin clotting time and the endogenous thrombin potential (ETP) is discussed, as well as the e vidence behind the accepted therapeutic ranges. The two most common te sts, PT and aPTT, respond in an essentially different way to clinicall y effective anticoagulation with heparin and with oral anticoagulants. This means that they covariate with, but do not themselves represent the essential parameter influenced by anticoagulation. The experimenta l basis for the widely accepted two times prolongation of the aPTT as an indicator for adequate anticoagulation is shown to be meagre in the case of unfractionated heparin and lacking for the other anticoagulan ts. Common sources for error in the interpretation of anti-factor Xa- and anti-thrombin activity of heparins are indicated. Extensive experi ence with new tests like the ecarin clotting time and the ETP is still lacking. On the basis of preliminary data and in view of the importan ce of the enzymatic action of thrombin in the pathogenesis of thrombos is, the ETP is considered a possible candidate for a common parameter to assess different types of anticoagulants.