Factor XII (Hageman Factor, named for the first patient identified) is involved in the contact phase of coagulation. An hereditary deficiency is uncommon and may go clinically undetected.
A homozygote patient will have a marked decrease in Factor XII activity (0 to 1% of normal), while heterozygous patients may have a mild to moderate decrease in activity.
Clinical findings:
(1) The patient may not have a history of excessive bleeding, even after trauma or surgery.
(2) The patient may have a history of thrombosis (myocardial infarction at a young age, pulmonary embolism, superficial thrombophlebitis, other).
(3) A woman may give a history of second trimester fetal loss.
(4) Family members are asymptomatic but may give a history of low factor XII activity or unexplained thromboembolism.
Laboratory findings:
(1) markedly prolonged aPTT
(2) normal PT and thrombin time tests
(3) normal levels of other coagulation factors
(4) Factor XII activity is decreased by a specific assay
(5) A patient with thrombosis may show defective fibrinolysis. Activated Factor XII helps to initiate fibrinolysis, which is impaired when Factor XII levels are sufficiently low.
The defect in fibrinolysis underlies the risk of thrombosis.
