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Evaluate a prolonged aPTT and detect specific coagulation factor XII activity.6,7
Direct Xa or thrombin inhibitor therapy may cause factitiously low results.
Factor XII along with prekallikrein and high molecular weight kininogen make up the contact activation system. These factors are necessary for clot formation in the activated partial thromboplastin (aPTT). In the test tube, factor XII is activated by contact with negatively-charged surfaces;8 however, deficiencies of these factors have no hemorrhagic consequence because physiologic clotting is activated by alternate paths that bypass the contact system.6,7
Factor XII is an 80 kilodalton single-chain proenzyme that is synthesized in the liver. Factor XII's plasma concentration is 30 mg/mL and half-life is about 50 hours. Factor XII deficiency is usually inherited in an autosomal recessive manner and heterozygous deficiency is relatively common, affecting somewhere between 1.5% and 3% of the population.6 In fact, mild factor XII deficiency is the most common cause of extended aPTT in the nonbleeding patient in the absence of lupus anticoagulant.6 Factor XII deficiency should be suspected whenever a patient has a normal protime (PT) and an extended aPTT and no history of bleeding. Factor XII levels are moderately diminished in heterozygous individuals with levels ranging between 20% and 60% of normal.7 Homozygous individuals typically have levels <1%.6 Severe factor XII deficiency is characterized by aPTT that can be longer than 100 seconds.7 Typically, there is correction with a normal plasma mixing study.
Factor XII can be affected, either increased and decreased in a number of conditions including septicemia, coronary artery disease, pharmacological thrombolysis, inflammatory bowel disease, pregnancy, lactic acidosis, hemodialysis, and angioedema.6,8 Decreased factor XII levels can be seen in liver disease and renal disease.6
A number of investigators have reported that congenital factor XII deficiency may be associated with an increased incidence of venous thrombosis;8 however, a recent consensus conference of the College of American Pathologists on diagnostic issues in thrombophilia found no evidence to support hypercoagulability in patients homozygously deficient for factor XII or any of the other contact factors.8
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; 107(1):105-110. PubMed 8980376
2. Reneke J, Etzell J, Leslie S, et al. 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; 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: 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; 107(6):681-683. PubMed 9169665
5. McGlasson DL, More L, Best HA, et al. Drawing specimens for coagulation testing: is a second tube necessary? Clin Lab Sci. 1999; 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. Kitchens CS. The contact system. Arch Pathol Lab Med. 2002; 126(11):1382-1386. PubMed 12421145
