A prothrombin time (PT) of 10 to 20 seconds is considered normal, indicating normal blood clotting. However, this raw PT result is rarely used in tracking blood coagulation over the long term. PT test results are adjusted using an international normalized ratio (INR), a mathematic ratio that adjusts for differences in the way labs conduct PT tests.

A healthy person would have an INR of 1.0. A higher INR/PT indicates that it takes blood longer to clot, while a lower number indicates that blood clots more quickly. Patients with atrial fibrillation are often told to maintain an INR of 2.0 to 3.0. Patients with artificial heart valves may be advised to maintain an INR of 2.5 to 3.5. Doing so reduces the risk of potentially fatal events such as a blood clot from the heart or from an artificial valve traveling to the brain (causing a stroke) to the lungs (causing a pulmonary embolism), to the legs (causing impaired limb circulation), to the kidneys and other areas of the body. 

Although each person’s case is different, abnormal or high INR readings may cause a physician to adjust the dosage or dose-schedule of warfarin. For instance, an INR between 3.5 and 5 may result in a recommendation to skip a dose of warfarin or reduction of the maintenance dose. At higher elevations, patients may be advised to skip warfarin and be given vitamin K. Vitamin K is an essential part of the coagulation cascade. Its administration has been shown to reverse the effect of excess anticoagulation faster than suspending warfarin alone. An INR greater than 20 should generally be treated aggressively to reduce the risk of hemorrhage.

A longer than normal international normalized ratio (INR) or prothrombin time (PT) in patients not taking anticoagulants means that the patient’s blood is taking too long to form a clot. Abnormal INR/PT results are frequently seen in patients with liver disease. Additional testing will usually be ordered to determine the specific cause.