Hereditary antithrombin III deficiency: biochemical aspects.

Since the description of the first thrombophilic family with congenital AT-III deficiency, increasing numbers of different types of the condition have become evident. Initially the anomaly seemed to be homogeneous and simple: the three main characteristics of AT-III (thrombin inactivating and heparin cofactor activity, antigen concentration) were decreased. This type of AT-III deficiency (type 1) was later divided into type 1a and 1b on the basis of the heparin affinity of the AT-III molecule. The first family with a different qualitative AT-III disorder (type 2) was described by our group in 1974. In the members of this family the AT-III antigen concentration was normal, but the molecule had no functional activity (AT-III Budapest). In the last few years some new variants of type 2 hereditary AT-III disorders have been observed; they are characterized by a loss of one or more functional properties of the AT-III molecule.