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Detecting antithrombin deficiency may be a difficult task – more than one test is necessary

Antithrombin (AT) is an important serine protease inhibitor that inhibits mainly coagulation factors IIa (FIIa) and Xa (FXa), but also FIXa and FXIa. AT deficiency is a well-known cause of thrombophilia [1,2] and therefore an obvious part of thrombophilia screening [3]. The activity of AT is usually quantitated by chromogenic tests measuring the inhibition of FIIa or FXa by AT in the presence of heparin (i.e. the heparin cofactor activity). In recommendations for thrombophilia tests, the type of AT activity assay is usually not recommended. Recently, however, focus on the importance of the type of assay has emerged. Ungerstedt et al. [4] described different levels in patients with AT deficiency using various Xainhibition methods, and in the United Kingdom National External Quality Assessment Scheme Jennings et al. [5] found varying results of certain AT deficiency variants depending on the assay conditions of the AT assays. We report on a case of AT deficiency in a patient who had had several thromboembolic attacks; the usual routine AT methods were unable to detect this, emphasizing the importance of the methodology.