Hirsh and Poller1 have examined a number of potential problems that may affect the reliability of the prothrombin time ratio and the international normalized ratio by highlighting sources of variability in the prothrombin time ratio and explaining how some of these are lessened by using the international normalized ratio. However, their article does not adequately emphasize the clinical impact that these inaccuracies have on patient care. The major concern in monitoring the intensity of anticoagulation therapy is how variability in either the prothrombin time ratio or the international normalized ratio affect the likelihood of adverse outcomes, bleeding and thromboemboli. Clinicians must separate variation in their patients' level of anticoagulation from variability introduced in the laboratory. By not accounting for variability in the sensitivity of thromboplastins to the depletion of the vitamin K—dependent coagulation factors, the prothrombin time ratio alone introduces needless uncertainty in the true level of anticoagulation intensity.