Control of Oral Anticoagulant Therapy with Whole Blood Prothrombin Time Devices: the Future Has Begun

Abstract

Whole blood prothrombin time (PT) devices (here called monitors) are portable instruments designed to detect clot formation after addition of a small drop of whole blood to a cartridge containing dry thromboplastin and calcium chloride. The PT can be automatically converted into INR and displayed by the monitor. During the last few years monitors have been increasingly proposed as substitute for conventional laboratory systems to control oral anticoagulant treatment (1-3). The distinct advantage is that they can be used for patient self-testing. Their use in combination with computer programs designed to adjust the dosage may open a new era in oral anticoagulant monitoring and all parts involved will benefit from such revolution. Physicians in charge (specialists and general practitioners) will be able to cope with the increasing numbers of patients on oral anticoagulants. Laboratory workers now engaged in such huge routine may be available for other tasks. Health services may save considerable amount of economic resources now spent in the management of patients on oral anticoagulants. Patients themselves may benefit by spending less time to attend overcrowded waiting rooms. However, the essential prerequisite for the use of monitors is that they meet the requirements set by the World Health Organization (WHO) for conventional systems with respect to the INR measurement. In this respect calibration is of paramount importance. Although no specific guidelines have so far been issued, it is provisionally accepted that monitors should be calibrated in the same way as conventional systems. Presently, the responsibility for calibration rests on manufacturers, who should ensure reliability of the calibration parameters such as the international sensitivity index (ISI) and the mean normal PT (MNPT) encoded in the cartridge. Independent attempts to check these parameters supplied by manufacturers have been done in few instances by expert laboratories (6-8). Calibration of monitors by unskilled users is presently not feasible because of the complexity of the procedure, the need for fresh whole blood, the need for international standards for thromboplastins and the skill to perform the manual PT testing which is required with international standards. Furthermore, access to the software to modify encoded parameters is problematic. Attempts to work out an easier calibration procedure are currently under way (European Concerted Action on Anticoagulation, ECAA).