Factor XII (FXII) deficiency was first discovered in 1955 and was initially named after John Hageman, the first patient diagnosed with the condition. The incidence of FXII deficiency is estimated at 1 in 1 million. This deficiency is inherited in an autosomal recessive fashion, which means it affects both sexes equally. It has been reported that FXII levels arelower among Asians, than any other ethnic group.
FXII deficiency is notable because despite gross abnormalities in laboratory tests of blood coagulation, patients with this condition do not bleed and normally do not require treatment. Having a low FXII level therefore has little or no clinical significance.
FXII is involved in the contact activation of blood coagulation. However, this activation pathway does not appear to play a significant role in vivo - hence the lack of bleeding tendency in those affected by FXII deficiency.
Most cases of FXII deficiency are diagnosed by chance, or during pre-surgical testing. The diagnosis is usually suspected on the basis of an extremely prolonged (typically >200 seconds) activated partial thromboplastin time (APTT) test and a normal prothrombin time (PT). The grossly abnormal APTT is nevertheless completely corrected by normal plasma in a mixing study. A specific FXII clotting assay is necessary to confirm the diagnosis.
