This test measures the heparin-dependent activity of ATIII utilizing a chromogenic assay that measures its ability to inhibit factor Xa. This test does not delineate between type 1 and type 2 deficiency. The ATIII Antigen test is used to identify type 1 deficiency thus determining whether the patient has type 1 or type 2.
ATIII is a potent inhibitor (anticoagulant) of multiple reactions in the coagulation cascade (Factors XIa, IXa, Xa, and IIa) and is one of the primary natural anticoagulant proteins. In type 1 ATIII deficiency the patients have an inadequate amount of antithrombin present in their circulation. ln type 2 deficiency, the amount of ATIII present is normal, but it does not function properly.