JACK B. ALPERIN, M.D.
To the Editor: In the paper entitled "Quinidine-Induced Hypoprothrombinemic Hemorrhage in Patients on Chronic Warfarin Therapy" (Annals of Internal Medicine, March 1968) Dr. Jan Koch-Weser is in error when he implies that factor IX levels influence the one-stage prothrombin time. Factor IX (Christmas factor) is not measured by the prothrombin time. Those factors involved in the prothrombin time include prothrombin, V, VII, X, and fibrinogen (1).
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ALPERIN JB. Christmas Factor (IX) and Warfarin Therapy. Ann Intern Med. 1968;68:1369–1370. doi: 10.7326/0003-4819-68-6-1369_2
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Published: Ann Intern Med. 1968;68(6):1369-1370.
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