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).
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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