PTT - A test used to evaluate the blood clotting. If there is a deficiency in a particular blood clotting factor, the time required for clotting will be lengthened. Patients with severe liver disease, Von Willebrand disease, or hemophilia will have an abnormal (prolonged) PTT test. Heparin, a medication given to thin the blood, will prolong the PTT, making this a useful test for regulating heparin dosing.