Very rare. Autosomal recessive transmission. Factor XII is a contact factor of the coagulation cascade activated by kallikrein and which plays a role in the transformation of factor XI in active factor XI (XIa). Factor XIIa does not intervene in the spreading phase of the clotting cascade that takes place on the surface of activated platelets. It causes an increase of the ACT but does not cause any abnormal bleeding.

Anesthetic implications: 

a common cause of increase of the ACT that has no clinical significance.

References : 

Updated: May 2019