How is antithrombin assayed?

By William Aird

  • 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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