Papers have documented that thrombin-based AT assays, such as Coatest AT, show an overestimation of AT activity in patients on heparin therapy due to the influence of heparin cofactor II. The FXa-based method provides more valid results in patients on heparin therapy. There is no influence from heparin cofactor II, a2-macroglobulin, or a2-antitrypsin. The AT FXa assay is a better discriminant between AT deficient and non-AT deficient individuals than the thrombin based assay.