High prothrombin time levels mean a patient's blood clotting time is longer than normal, states Mayo Clinic. Average prothrombin times range between 10 and 14 seconds. Results for patients taking warfarin, a blood thinner, are stated as the international normalized ratio, INR; the range is about 2.0 to 3.0.
If an INR is above the range, the patient's blood is clotting too slowly. Doctors only use the INR for patients taking oral blood thinners, notes Mayo Clinic. Causes of high prothrombin time include blood thinners such as heparin, liver disorders, vitamin K deficiency and deficiencies in proteins required for clotting. The prothrombin time test aids in diagnosing liver or bleeding problems.