The Antithrombin III (AT III) test measures levels and activity of this crucial protein that inhibits blood clotting, helping diagnose inherited/acquired deficiencies (Type 1: low amount; Type 2: poor function) that raise risk for dangerous clots (DVT, PE). Doctors order it for unexplained clots, family history, pregnancy issues, or before surgery, using blood drawn into a blue-top tube (citrate) to check for clotting disorders or monitor anticoagulant therapy. Abnormal results (low activity) signal increased clotting risk, while high levels are less common but can link to inflammation or liver issues