Factor XII clotting activity and antigen levels in patients with thromboembolic disease.

Individuals with severe factor XII (FXII) deficiency may be prone to thromboembolic disease and this thrombophilic state may be due to insufficient contact activation dependent fibrinolysis. According to our previous study (Thromb Haemostas 1991; 65: 117-121), however, heterozygous FXII deficiency is not a strong prethrombotic risk factor, only one out of 45 obligatory or possible heterozygotes having sustained a thrombotic event. In the present study, FXII clotting activity (FXII:C) and antigen concentration (FXII:AG) were measured in 200 patients having suffered from idiopathic thromboembolism and compared with the values in 200 healthy controls. Mean FXII levels were not significantly different in thrombophilic patients and controls, and subnormal FXII values were not more frequently encountered in patients than in controls. Specific FXII activity, i.e. the ratio of FXII:C to FXII:AG, showed considerable variation in each of the two groups, but patients and controls had a similar distribution of specific FXII activity. Variations in specific FXII activity were not explained by differences in beta 2-glycoprotein I levels. In conclusion, heterozygous FXII deficiency is not a strong prethrombotic risk factor and subnormal FXII values are not more common in thrombophilic patients than in healthy individuals.