Quality control and oral anticoagulation.

The introduction of the INR system for the monitoring of oral anticoagulant control represents a major advance, not only in terms of therapeutic efficacy but also in respect of patient safety. It is important to recognize that the system is only viable if careful attention is paid to the many important variables which contribute to its overall value. These include the choice of thromboplastin, assignation of ISI, determination of the mean normal prothrombin time and the method of end-point detection. Participation in independent external quality assessment schemes permits a unique opportunity for individual laboratories to identify, through laboratory performance analysis, problems relating to their own laboratory practice. Another major advantage, particularly of the larger schemes, is the identification of poor reagents and coagulometers and inconsistencies in their performance. One example of this is the recent identification of problems in respect of ISI assignment for thromboplastins used with coagulometers. This, in turn, provided the necessary stimulus to the potentially important development of calibrants for local ISI determination (21).