Antithrombin (AT) Activity details

Antithrombin is a protein made by the liver that acts as one of your body's natural 'brakes' on blood clotting, inhibiting several clotting factors to help keep clot formation in check. This test measures how well antithrombin is functioning, and low activity — whether from an inherited deficiency or acquired causes like liver disease, nephrotic syndrome, or blood clots themselves consuming antithrombin — is linked to an increased risk of forming abnormal blood clots (thrombosis). It's typically ordered when someone has had an unexplained blood clot, especially at a young age or with a family history of clotting problems, as part of a broader clotting disorder ('thrombophilia') workup alongside tests like Factor V Leiden and protein C and S.