Large and small blood clots often form in the vessels of cats with heart disease. When these clots block major blood vessels, they cause extreme pain and distress to the cat because tissues die from lack of blood and oxygen. Many cats do not survive an initial clotting episode. Although we know cats with certain types of cardiac diseases are at risk of abnormal blood clots formation, we have no laboratory tests to detect these clots before they cause severe tissue injury or to determine which cats are at risk of these cats. If such cats were identified, they could be treated with anti-clotting (anti-coagulant) drugs to prevent clot formation. In this study, researchers will examine whether two laboratory tests of clot formation commonly used in people, thrombin-antithrombin complexes and D-dimer, can be applied to cats. These assays are used for diagnosing and monitoring human patients with thrombosis, but have not been tested in cats. If these tests prove to be useful in cats, they could be used by veterinarians for the early detection of clots, to identify cats that might benefit from anti-coagulant drugs, and to develop safe and effective treatment protocols to prevent abnormal clot formation.