- Functional assay:
- Assay of choice for screening
- Amidolytic (chromogenic):
- Patient’s plasma is incubated in the presence of heparin with excess thrombin or FXa.
- The AT in the patient’s plasma reacts with and neutralizes thrombin or FXa, a reaction catalyzed by heparin.
- The amount of thrombin or FXa that remains non-neutralized is inversely proportional to the patient’s AT activity level, and this remaining thrombin or FXa is then quantified using an automated chromogenic detection system.
- Best performed several days after cessation of heparin therapy, as acute thrombosis and heparin therapy may transiently lower AT levels.
- Antigen assay:
- No need to perform routinely an AT antigen assay, either as a screening test or as an additional test, if the AT activity is normal.
- A normal AT antigen level does not rule out a type II deficiency, which is more common than type I.
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