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Subject: Factor XII (Hageman Factor)
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Factor XII is synthesized in the liver. It circulates in an inactive form. It is activated by collagen, disrupted basement membranes, and activated platelets, as well as by high molecular weight kininogen and prekallikrein in conjunction with factor XI. It is unaffected by vitamin K antagonists.
Normal range: 60–150%.
Specific factor assay is needed for the diagnosis of factor XII deficiency and to distinguish the anomaly from factor XI or other intrinsic pathway initiating factors deficiencies.
PTT, but not PT, is prolonged in severe deficiency.
Asian populations have lower factor XII levels than Caucasians (average, 44%).
Factor XII levels are decreased in the neonate; they reach adult values at 2 weeks of age.
Factor XII levels are increased in pregnancy.
Factor XII may be artifactually decreased by the presence of LA.