Factor XI Activity (Antihemophilic Factor C / Plasma Thromboplastin Antecedent (PTA))

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Turnaround Time: 2 - 3 days
CPT Code:


Test Type: 1 mL Plasma, frozen


Evaluate isolated aPTT prolongation and to document specific factor deficiency.6-8

Factor XI activity is determined utilizing an aPTT-based one-stage clotting time assay. Factor XI-depleted plasma is used as the substrate, and the clotting time with the patient plasma is compared to the clotting time of normal pooled plasma.

Factor XI is a 160 kilodalton glycoprotein proenzyme that is produced by the liver and megakaryocytes.6-8 Factor XI's plasma concentration is 4-6 mg/mL and half-life is about 60 hours.6 Hereditary factor XI deficiency, referred to as hemophilia C, is transmitted as an autosomal recessive mutation.6-8 This condition affects both males and females and the majority of reported cases have been diagnosed in Ashkenazi Jews.6,7 As many as 11% of Ashkenazi Jews will be heterozygous for factor XI deficiency and up to 0.3% will be homozygous.8 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.7 Severity of bleeding does not always correlate with the plasma level of factor XI.6,7 Individuals with factor XI deficiency can suffer from easy bruising, epistaxis, hematuria, and menorrhagia.6,7 Excessive bleeding postpartum and after oral cavity surgery can occur.7

Acquired inhibitors of factor XI are very rare.6 Spontaneous autoantibodies are more common and generally occur in patients with underlying autoimmune disorders or in patients treated with chlorpromazine.6

1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997 Jan; 107(1):105-110. PubMed 8980376

2. Reneke J, Etzell J, Leslie S, Ng VL, Gottfried EL. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998 Jun; 109(6):754-757. PubMed 9620035

3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).

4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997 Jun; 107(6):681-683. PubMed 9169665

5. McGlasson DL, More L, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999 May-Jun; 12(3):137-139. PubMed 10539100

6. Adcock DM, Bethel MA, Macy PA. Coagulation Handbook. Aurora, Colo: Esoterix-Colorado Coagulation; 2006.

7. Roberts HR, Escobar MA. Less common congenital disorders of hemostasis. In: Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002: 57-71.

8. Triplett DA. Coagulation abnormalities. In: McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Philadelphia, Pa: Lippincott Williams and Wilkins; 2002:1033-1049.

Collection Details:

Patient Preparation:

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.

Collection Instructions:

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted.

Blue-top (sodium citrate) tube.

Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood-to-anticoagulant ratio.2,3 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.4,5 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.