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Subject: Factor XI
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Factor XI is synthesized in the liver and megakaryocytes. Factor XI is activated by factor XIIa and by thrombin, the preferred activator on platelet surface. In turn, factor XI activates factors XII and IX in the intrinsic pathway. Factor XI is not affected by vitamin K antagonists.
Normal range: 60–120%.
For the diagnosis of factor XI deficiency, a specific functional assay to quantitate the factor has to be performed.
If factor XI is decreased to <20–25%, PTT, but not PT, is prolonged. A normal PTT does not rule out a mild factor XI deficiency.
Antibody inhibitors develop relatively often as a consequence of replacement therapy in factor XI–deficient patients.
Decreased values are characteristic for patients with factor XI deficiency. Acquired low values occur in severe liver disease and DIC.
High levels of factor XI have been recently shown to be a risk factor for venous thromboembolism.