Since the Prothrombin time (PT) evaluates the ability of blood to clot properly, it can be used to help diagnose bleeding. When used in this instance, it is often used in conjunction with the PTT to evaluate the function of all coagulation factors. Occasionally, the test may be used to screen patients for any previously undetected bleeding problems prior to surgical procedures.

The International Normalized Ratio (INR) is used 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/INR is appropriately prolonged. There is no set frequency for doing the test. Your doctor will order them often enough to make sure that the drug is producing the desired effect - that it is increasing your clotting time to a therapeutic level without causing excessive bleeding or bruising.

The PT may be ordered when a patient who is not taking anti-coagulant drugs has signs or symptoms of a bleeding disorder, which can range from nosebleeds, bleeding gums, bruising, heavy menstrual periods, blood in the stool and/or urine to arthritic-type symptoms (damage from bleeding into joints), loss of vision, and chronic anemia.

Sometimes the PT may be ordered when a patient is to undergo an invasive medical procedure, such as surgery, to ensure normal clotting ability.

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) for patients on anti-coagulant drugs. These patients 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.

If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause. A prolonged, or increased, PT means that your blood is taking too long to form a clot. This may be caused by conditions such as liver disease, vitamin K deficiency or a coagulation factor deficiency. Result of the PT is often interpreted with that of the PTT in determining what condition may be present.

Interpretation of PT and PTT in patients with a bleeding syndrome

PT Result	PTT Result	Possible Condition Present
Prolonged	Normal	Liver disease, decreased vitamin K, decreased or defective factor VII
Normal	Prolonged	Decreased or defective factor VIII, IX or XI or lupus anticoagulant present
Prolonged	Prolonged	Decreased or defective factor I, II, V or X, von Willebrand disease, liver disease, disseminated intravascular coagulation (DIC)
Normal	Normal	Decreased platelet function, thrombocytopenia, factor XIII deficiency, mild deficiencies in other factors, mild form of von Willebrand’s disease

Some substances you consume, such as alcohol, can affect the PT/INR test. Some antibiotics 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 your PT/INR results are interpreted and used correctly.