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The Quick (1) and Owren(2) prothrombin time (PT) tests remain the basis for monitoring anticoagulant therapy worldwide. The use of these tests is remarkable because the Quick PT preceded the discovery of the anticoagulant effect of dicoumarol in spoiled sweet clover and both tests were in use long before there was any understanding of the mechanism of the action of oral anticoagulants or which of the plasma factors were affected by them. Both PT tests are conceptually based on the four-factor theory of coagulation proposed by Morawitz in 1905 (3), and the two tests are similar. The reactions of the Quick PT test depend entirely on coagulation proteins present in the patient plasma sample; the Owren PT test adds components of bovine plasma to compensate for variability related to the coagulation proteins fibrinogen and factor V. Efforts to standardize the Quick PT have provided a common means for expressing patient PT results, the International Normalized Ratio (INR), and have provided a method for quantitative comparison of thromboplastin reagents (4). These efforts have improved interlaboratory comparability of mean values and standard deviations for groups of patient samples.In this issue of Clinical Chemistry , Horsti et al. (5) report PT results from testing the same samples, from patients receiving oral anticoagulants, on the same instruments but with different thromboplastins. They demonstrate that intrapatient comparability has not been achieved by use of the current standardization procedures (4). Their systematic comparison of seven different thromboplastins provides compelling evidence that neither the Quick nor the Owren PT test provides suitably comparable data for monitoring individual patients receiving oral anticoagulants if more than one thromboplastin reagent is used. Direct comparisons of the INR results from an individual patient’s samples measured with use of different thromboplastins indicate that the INR values can vary dramatically, certainly …

Has the Time Arrived to Replace the Quick Prothrombin Time Test for Monitoring Oral Anticoagulant Therapy?