Prevalence of antithrombin III deficiency in blood donors selected for personal or familial history of venous thrombosis.

Antithrombin III (ATIII) was measured using a functional assay in 692 (6.7%) out of 10,332 blood donors selected for their personal or familial history of venous thrombosis. Three subjects with low levels of the protein were observed. Thus, the prevalence of ATIII deficiency was of 0.43%, corresponding to a prevalence of 0.03% in the general population. On the basis of family studies and of clinical and laboratory investigations, inherited ATIII deficiency was excluded and reduced levels of the inhibitor were attributed to an impaired liver function, despite normal blood coagulation tests. Therefore, in this study inherited ATIII deficiency cases were not found; however, acquired ATIII deficiency associated with normal clotting tests might represent a rare but definite risk factor for thrombosis.