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A Simple Method for Selecting the Dose of Warfarin

Jacob Zatuchni, MD
JAMA. 1988;259(17):2543-2544. doi:10.1001/jama.1988.03720170019016.
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To the Editor.  —Regarding the article by Drs Hirsh and Levine entitled "The Optimal Intensity of Oral and Anticoagulant Therapy,"1 I point out that the international normalized ratio system is not in wide use. There remains confusion about translation of results to estimating the dosage of warfarin. It is clear that anticoagulant control as prescribed by hospital physicians who apparently look at daily prothrombin times results in levels within the recommended range no more frequently than could be achieved by chance.2For these reasons, I wonder if Drs Hirsh and Levine would agree to a simple method for selection of warfarin dosage (Table). It is based on the prothrombin time reported by the laboratory in which a prothrombin time ratio of 1.3 to 1.5 corresponds to 14 to 15 s and that of 1.5 to 2.0 to 18 to 20 s, corresponding to an international normalized ratio of 2.0


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