The Antithrombin III (ATIII) test measures the activity or amount of ATIII protein, a natural blood thinner, to assess your risk for excessive clotting (thrombosis) or bleeding, often used for unexplained clots (like DVT, PE), recurrent miscarriages, or before surgery. A deficiency, where levels are low or the protein doesn’t work well, increases clotting risk, while high levels usually aren’t a problem. It’s a simple blood draw from a vein in your arm, done to find the cause of clotting issues and manage treatment