Guidelines for warfarin dosage.

are suggested. 10 The suggestions are empirical and are to be modified depending on desired goals and response of the patient. A PT of 14-15 sec corresponds approximately to a PT ratio of 1.21.5, and a PT of 18-20 sec to a PT ratio of 1.5-2.0. In turn, any PT ratio can be converted to the respective INR value obtained from Table I and adjustments made in the dose of warfarin if necessary. In such a manner, important variability in the PT ratio caused by different brands and batches of thromboplastin and instruments may be accounted for; also, the INR scale will facilitate the process of determinating optimal therapeutic ranges.",12 Finally, it is also important to note that PT is timeand temperature-dependent. Spurious shortening may occur when blood is in contact with glass surfaces for longer than one hour and may lead to unnecessarily larger doses of warfarin." Also, potential complications may occur if different preparations of warfarin are employed. I' The warfarin doses suggested above are based on daily results of prothrombin time for inpatients. For outpatients, a reasonable determination is possible based on analysis of dose employed and response, with PT obtained at frequent intervals initially and at longer intervals later, preferably every 3 weeks and perhaps as long as every 12 weeks." 10.0 7.5 5.0 2.5 o WARFARIN DOSE (rng)