Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (918 page)

   
Normal range:
60–120%.

   Use

   For the diagnosis of factor XI deficiency, a specific functional assay to quantitate the factor has to be performed.

   Interpretation

   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.

FACTOR XII (HAGEMAN FACTOR)
^†

   Definition

   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%.

   Use

   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.