A prothrombin time (PT) test is a type of blood test that measures how long it takes for a blood clot to form in a blood sample after the blood has been exposed to a specific reagent, or clotting factor. This test is frequently used to monitor patients who are on anticoagulant therapy.

However, due to differences in the reagents used, as well as differences in lab equipment, PT test results may vary widely from lab to lab. Some reagents cause blood to clot more quickly than others, while certain equipment may speed or slow clotting time. To correct this, in 1983 the World Health Organization developed the INR, or international normalized ratio. This ratio allows different labs to standardize their PT test results. Thus, PT test results from one lab can be compared to PT test results from any other lab in the world.

The INR is figured by dividing the raw patient PT score by a standard control PT score, multiplied by a figure known as the ISI (international sensitivity index).

The ISI was derived by using an international reagent, although each lab will calculate a different ISI based on their specific equipment and reagent combination. Similarly, the control PT is unique to each lab – it is that lab’s mean normal PT, based on a large sample of blood tests that are handled identically to the typical patient test.

Using the INR/PT, physicians are able to more accurately monitor and compare the blood clotting time of patients on anticoagulant medication, specifically warfarin therapy. The INR/PT does not apply to patients taking heparin, another anticoagulant, because heparin does not slow the PT. Nor is it applicable to patients with liver disease or antiphospholipid syndrome. The INR/PT allows patients on anticoagulant therapy to travel from country to country or state to state and still monitor their therapy.

INR/PT takes only a few minutes and is relatively inexpensive, making it popular both for diagnosing clotting disorders and monitoring the progress of patients taking anticoagulants (medications that prevent the formation of potentially dangerous blood clots). The test can be performed by a physician, nurse or certified technician in a hospital, physician’s office or clinical laboratory.

Patients taking anticoagulants will need to undergo INR/PT testing regularly to monitor their medication dosage. These tests usually require the patient to visit a hospital, physician’s office or clinical laboratory monthly or weekly.