Evaluation of the clotting defect in a factor XII-deficient kindred

A family with factor XII severe congenital deficiency is described. Factor XII activity and factor XII antigen were both undetectable in the propositus plasma; levels of FXII:C and FXII:Ag were intermediate in heterozygotes. Plasma prekallikrein activity was low in the propositus, whereas normal levels of antigen could be found, suggesting a defect of kallikrein activation due to factor XII deficiency.