Factor XII
Hereditary deficiency of factor XII (Hageman factor) is usually diagnosed incidentally when a patient presents with an unexplained prolongation of the APTT, often during preoperative coagulation screening. There is general agreement that even severe factor XII deficiency is not associated with a clinical bleeding tendency. More controversial, however, is the role of factor XII deficiency as a possible prothrombotic risk factor. This is based on several case reports of venous thrombosis or myocardial infarction in patients with severe factor XII deficiency. Furthermore, several cohort studies of patients with venous or arterial thrombosis have suggested a higher than expected prevalence of factor XII deficiency, although case-control studies have failed to confirm this.
A recent study was undertaken to reevaluate the role of factor XII deficiency as a possible thrombophilic risk factor. Sixty-five subjects from 12 Swiss families with factor XII deficiency were studied. The occurrence of venous or arterial thrombosis was documented, as well as factor XII levels and factor XII genotype. The results of the study indicate that both heterozygous (partial) and homozygous (severe) factor XII deficiency are very unlikely to be prothrombotic risk factors. Furthermore, none of the specific factor XII gene defects identified in these families was associated with thrombosis. Despite the participation of factor XII in both the intrinsic coagulation cascade and the fibrinolytic pathway, it appears that hereditary factor XII deficiency is an asymptomatic defect.
