International standardization of laboratory control of oral anticoagulant therapy: a survey of thromboplastin reagents used for prothrombin time testing

Abstract

The prothrombin time (PT) test is the primary measurement in the laboratory control of oral anticoagulant treatment. The traditional expression of PT test results, either as percentage prothrombin activity or PT ratio, is inadequate for international communication and comparison because the values depend on the nature of the thromboplastin test system used. The WHO recommended universal scale of reporting PT results is based on calibration of local thromboplastin systems against an international reference preparation. This scale is the International Normalized Ratio (INR). Application of the INR scale in clinical practice should be encouraged by External Quality Assessment (EQA) schemes, which improve its precision. Many physicians are insufficiently aware of the different sensitivities of rabbit thromboplastins, which result in different anticoagulation intensities being employed in different clinics. Improvement in the situation can only be achieved by continuous education. The more widespread use of the INR scale should facilitate international comparison of anticoagulation results and eventually consensus on optimal target values. The introduction of the INR to countries not already using it, particularly the USA, should be strongly encouraged by all physicians with an interest in anticoagulation and especially those undertaking the long-term management of patients with prosthetic heart valves.