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October 24, 1977

Factor XI Deficiency

JAMA. 1977;238(17):1811. doi:10.1001/jama.1977.03280180015007

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To the Editor.—  The article by Jokl and Federico entitled "Myositis Ossificans Traumatica: Association with Hemophilia (Factor XI Deficiency) in a Football Player" (237:2215-2216, 1977) is not adequately documented as a case of factor XI deficiency.Factor XI assays in their patient were determined to be 46% and 52%. The normal range is 50% to 185%. The dividing line between homozygotes and heterozygotes is estimated at only 10% to 20%. Before labeling this patient as having factor XI deficiency, several questions must be answered: (1) What is the upper limit of normal of the partial thromboplastin time in their laboratory? (2) What activator was used in the performance of the partial thromboplastin time? (3) Were substitution studies performed using factor XI substrate plasma? (4) Were studies performed to exclude the presence of a circulating anticoagulant? (5) Is the patient of Jewish ancestry?In that the minimal hemostatic level of factor