Platelet antithrombin deficiency: a new clinical entity.

A kindred with a history of multiple thromboses was studied for coagulant abnormalities. A deficiency of serum antithrombin III was found in approximately half of the 13 family members by either coagulant or immunologic assay. No clear relationship between antithrombin III deficiency and a history of thrombosis was present. Platelet antithrombin assays were also studied in the same subjects. Ten of the 13 members were deficient. None of the remaining three had a history of thrombosis. On the basis of these findings, the hypothesis is proposed that in some cases familial hypercoagulability may be due to a platelet antithrombin deficiency and that the serum antithrombin III deficiency in some cases is a secondary rather than a primary effect.