EFFECT OF LABORATORY VARIATION IN THE PROTHROMBIN-TIME RATIO ON THE RESULTS OF ORAL ANTICOAGULANT-THERAPY

Background. Patients receiving long-term anticoagulant therapy may be subject to unnecessary risks of bleeding or thromboembolism because of variability in the commercial thromboplastins used to determine prothrombin time and consequent uncertainty about the actual intensity of anticoagulation. Methods. We explored the effect of this uncertainty on the benefits and risks of anticoagulation in patients with prosthetic heart valves, using models of thromboembolic and hemorrhagic complications as a function of the intensity of anticoagulation, with quality-adjusted life expectancy and average variable costs used to describe outcomes. Results. Anticoagulation provides a striking benefit for patients whose treatment is conducted within the recommended range of the international normalized ratio (INR) - i.e., 2.5 to 3.5 - but if uncertainty about the laboratory results causes the intensity of anticoagulation to fall outside this range, the gain becomes smaller. Uncertainty about the true intensity of anticoagulation may reduce the potential gain in life expectancy, adjusted for quality of life, by more than half and may increase the ratio of costs to effectiveness to almost five times the optimal value. Variability in the intensity of anticoagulation is even greater if older recommendations advocating a higher level of anticoagulation are followed. Conclusions. Uncertainty about the sensitivities of the commercially available thromboplastins used in the United States can have important clinical and economic effects. This problem could be eliminated if clinical laboratories uniformly reported the intensity of anticoagulation as the INR, by adjusting prothrombin-time ratios for variability in thromboplastins.