Effect of Laboratory Variation in the Prothrombin-Time Ratio on the Results of Oral Anticoagulant Therapy

The more than half a million Americans receiving long-term anticoagulant therapy face unnecessary risks of bleeding or thromboembolism because of variability in the commercial thromboplastins used by clinical laboratories to determine prothrombin times. There were marked disparities in the sensitivity index of the thromboplastins used by the 53 laboratories involved in the Stroke Prevention in Atrial Fibrillation (SPAF) study 1 . This means that although two laboratories report the same prothrombin-time ratio, the intensity of anticoagulation may differ substantially. Put another way, for the same specimen analyzed at two different laboratories, markedly different prothrombin-time ratios could be reported. If such variability . . .