Factor XII deficiency

Factor XII deficiency is predominantly a laboratory finding rather than a clinical disease. Most affected individuals are entirely asymptomatic, and the condition is typically detected incidentally during preoperative coagulation screening or evaluation of an unexplained prolonged aPTT. Severe deficiency produces a markedly prolonged aPTT but does not cause spontaneous or surgical bleeding, distinguishing it clinically from haemophilia and other intrinsic-pathway factor deficiencies with clinical bleeding tendencies. The disorder is congenital, with the abnormal laboratory phenotype being present from birth. Clinical recognition occurs at any age, depending on when coagulation testing is first performed. Beyond the laboratory abnormality, FXII deficiency has been linked in observational studies to thrombotic events and recurrent pregnancy loss. However, these associations remain controversial and ultimately unproven.

The exact incidence of the disorder in the general population is unknown but estimated to be approximately 1 in 1 million individuals.

FXII deficiency is caused by germline pathogenic variants in F12, which encodes coagulation factor XII (Hageman factor). Severe deficiency requires biallelic (homozygous or compound heterozygous) pathogenic variants; heterozygous carriers typically retain partial (40%-60%) FXII activity. Reported pathogenic variants include missense substitutions, splice-site variants, small deletions, and frameshift changes; these variants reduce FXII antigen levels, coagulant activity, or both, impairing initiation of the contact activation pathway. FXII deficiency can also be secondarily acquired. Among the possible causes are nephrotic syndrome, liver disease, sepsis and shock, and disseminated intravascular coagulation.

While loss-of-function variants in F12 lead to FXII deficiency, distinct gain-of-function variants located primarily in exon 9 of the F12 gene cause autosomal dominant hereditary angioedema with normal C1 inhibitor (OMIM #610618). These variants render FXII hyper-susceptible to premature cleavage by plasmin, triggering excessive bradykinin release and severe episodic angioedema.

FXII deficiency is inherited in an autosomal recessive manner. The recurrence risk for siblings of an affected individual born to two carrier parents is 25%, and offspring of an affected individual will themselves be obligate carriers unless the reproductive partner also carries an F12 pathogenic variant. Prenatal molecular testing is technically possible once the familial pathogenic variants have been identified.

FXII deficiency is diagnosed by a specific coagulant activity assay, typically prompted by detection of an isolated prolonged activated partial thromboplastin time with a normal prothrombin time. Correction of the prolonged aPTT on a 1:1 mixing study with normal pooled plasma supports a factor deficiency rather than an inhibitor; severe deficiency is confirmed by FXII antigen measurement and molecular analysis of the F12 gene.

• Establishing the molecular diagnosis confirms the clinical suspicion and provides a defined aetiology for the affected individual.
• Identification of the familial pathogenic variant enables targeted testing of at-risk relatives and supports reproductive counselling where relevant.
• Identifying severe FXII deficiency prevents unnecessary transfusions or procedure cancellations driven by an isolated prolonged aPTT.

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• Stępnicki J, Imiela AM, Szymańska M, Mikołajczuk J, Pruszczyk P. Factor XII in Thrombosis and Thromboinflammation: From Molecular Biology to Clinical Translation. International journal of molecular sciences. 2026;27(7). doi: 10.3390/ijms27073336. PMID: . PMCID: PMC13073903.
• Chou SC, Lin CY, Lin HY, Pai CH, Yu CY, Kuo SF, et al. Characterization of congenital factor XII deficiency in Taiwanese patients: identification of one novel and one common mutation. International journal of hematology. 2022;116(4):528-533. doi: 10.1007/s12185-022-03390-0. PMID: . PMCID: PMC9174919.