The antithrombin activity and antigen tests are used to help find out what may be causing abnormal blood clots in your body. A blood can be good or bad, depending on the case. Antithrombin is a protein produced by the liver that helps regulate blood clot formation. Normally you have a healthy balance between chemicals in your body that help clotting and chemicals that stop clotting. One important protein that helps clotting is thrombin. The protein that blocks thombin is called antithrombin. Antithrombin works to thin the blood slightly so that it doesn't clot too much. A lack of antithrombin can make it more likely for you to form blood clots.