Since PT and INR (PT/INR) evaluate the ability of blood to clot properly, they can be used to assess both bleeding and clotting tendencies. One common use is to monitor the effectiveness of blood thinning drugs such as warfarin (Coumadin). These anti-coagulant drugs help inhibit the formation of blood clots. They are prescribed on a long-term basis to patients who have experienced recurrent inappropriate blood clotting. This includes those who have had heart attacks, strokes, and deep vein thrombosis (DVT). Anti-coagulant therapy may also be given as a preventative measure in patients who have artificial heart valves and on a short-term basis to patients who have had surgeries, such as knee replacements. The anti-coagulant drugs must be carefully monitored to maintain a balance between preventing clots and causing excessive bleeding.

If you are taking an anti-coagulant drug, your doctor will check your PT/INR regularly to make sure that your prescription is working properly and that your PT is appropriately increased. There is no set frequency for doing the test. Your doctor will order them often enough to make sure that the drugs are creating the desired effect - that they are increasing your clotting time to a therapeutic level without causing excessive bleeding or bruising.

The PT test may be ordered on a patient who is not taking anti-coagulant drugs to check for a bleeding disorder or to ensure clotting ability before surgery.

The test result for PT depends on the method used, with results measured in seconds and compared to the average value in healthy people. Most laboratories report PT results that have been adjusted to the International Normalized Ratio (INR). Patients on anti-coagulant drugs should have an INR of 2.0 to 3.0 for basic “blood-thinning” needs. For some patients who have a high risk of clot formation, the INR needs to be higher - about 2.5 to 3.5. Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you. A prolonged, or increased, prothrombin time means that your blood is taking too long to form a clot. If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause.

Some substances you consume, such as alcohol, can affect the PT/INR test. Antibiotics, aspirin, and cimetidine can increase the PT/INR. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT. Certain foods (such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens, and soybean products) contain large amounts of vitamin K and can alter PT results. Make sure that your doctor knows all the drugs you are taking and if you have eaten any of these foods recently so that the PT/INR results are interpreted and used correctly.