TO THE EDITOR:

Factor XI is a zymogen of a serine protease that participates in the intrinsic pathway of coagulation. Deficiency of factor XI, first described in 1953, is characterized by mild to moderate bleeding after injury in tissues with high fibrinolytic activity such as the oral and nasal cavities and the urinary tract. Spontaneous bleeding is rare in factor XI–deficient patients. Treatment of factor XI deficiency is a challenge, because administration of fresh frozen plasma may lead to volume overload, and factor XI concentrates are not generally available. There are 2 plasma-derived concentrates, Hemoleven (a high-purity product from LFB Biomedicaments, France) and FXI concentrate (a partially purified product from Bioproducts Laboratory, United Kingdom), that have shown efficacy but also demonstrated safety concerns.3,4  Traces of factor XIa in the concentrates were thought to lead to thrombosis and disseminated...

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