Factor XI Activity
Antihemophilic Factor C
Plasma Thromboplastin Antecedent (PTA)
Evaluate isolated aPTT prolongation and to document specific factor deficiency.
Factor XI is a 160 kilodalton glycoprotein proenzyme that is produced by the liver and megakaryocytes. Factor XI’s plasma concentration is 4-6 mg/mL and half-life is about 60 hours. Hereditary factor XI deficiency, referred to as hemophilia C, is transmitted as an autosomal recessive mutation. This condition affects both males and females and the majority of reported cases have been diagnosed in Ashkenazi Jews. As many as 11% of Ashkenazi Jews will be heterozygous for factor XI deficiency and up to 0.3% will be homozygous. Individuals who are heterozygous for factor XI deficiency mutation typically have levels between 30% to 60% and homozygotes have levels <20%.8 The bleeding associated with factor XI deficiency is generally not as severe as that found with hemophilia A or B. Severity of bleeding does not always correlate with the plasma level of factor XI. Individuals with factor XI deficiency can suffer from easy bruising, epistaxis, hematuria, and menorrhagia. Excessive bleeding postpartum and after oral cavity surgery can occur.
Acquired inhibitors of factor XI are very rare. Spontaneous autoantibodies are more common and generally occur in patients with underlying autoimmune disorders or in patients treated with chlorpromazine.
Blue-top (sodium citrate) tube
Ideally, the patient should not be on anticoagulant therapy. Avoid warfarin (Coumadin®) therapy for two weeks prior to the test and heparin, direct Xa, and thrombin inhibitor therapies for about three days prior to testing. Do not draw from an arm with a heparin lock or heparinized catheter.
Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate. Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio. The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples unless the sample is collected using a winged (butterfly) collection system. With a winged blood collection set a discard tube should be drawn first to account for the dead space of the tubing and prevent under-filling of the evacuated tube. When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternative anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes.
Severe hemolysis; improper labeling; clotted specimen; specimen diluted with IV fluids; samples thawed in transit; improper sample type; sample out of stability