Homozygous or compound heterozygous qualitative antithrombin III deficiency.

A male patient of 24 years who had experienced thrombotic episodes since the age of 15 displayed an unusually low antithrombin III (AT III) activity measured as heparin cofactor (13% of the normal), while a similarly decreased value (16% of normal) was found in a 26 year old brother who had suffered from thrombotic events since the age of 12 years. AT III heparin cofactor activities were close to 50% of normal in the father, mother, another brother and a sister, none of whom had experienced thrombotic episodes. Since all available members of the family, including the patient, displayed near normal AT III antigen levels (73-85%) normal total progressive antithrombin activities (92-110%) as assessed by the thrombin agarose diffusion technique and normal total progressive anti-Xa activities, the propositus and his brother could be considered to be homozygotes or compound heterozygotes for a qualitative familial AT III deficiency probably caused by an abnormality of the heparin binding site. Molecular techniques would be required to elucidate the precise mutation giving rise to the deficiency.