Antithrombin III deficiency.

A moderate reduction of plasma antithrombin activity is an uncommon but clinically important cause of severe thromboembolic disease. In recent years the molecule responsible for the major part of this activity (antithrombin III) has been extensively characterised and the mode of inheritance of familial deficiencies worked out. Over 30 autosomally dominant inheritable variants have been described, the gene for normal human antithrombin III has been sequenced and this information has provided important insights into the reaction of antithrombin with thrombin and the catalytic role of heparin. Further information has been derived by analogy with other serine proteinase inhibitors, in particular alpha 1 antitrypsin. Recombinant DNA methods have been used to produce functionally active AT III which may, in the future, replace human chromatographically-separated AT III as the treatment of choice for clinically important deficiency states. Newer diagnostic techniques, using restriction fragment length polymorphisms and synthetic oligonucleotide probes, hold the promise of more accurate diagnosis and diagnosis in the antenatal period in families possessing some of the fully characterised variants.