Prothrombin time or international normalized ration is a blood test that determines how long blood takes to clot, according to MedlinePlus. In most cases, this test is necessary when a doctor wants to monitor the effects of warfarin, which is a blood-thinning medication.
Other than checking the effects of warfarin, a PT test is useful when determining the cause of abnormal bleeding or bruising and for checking the efficiency of the liver functions, says MedlinePlus. It is also applicable in the diagnosis of bleeding or clotting disorder signs.
To perform a PT test, a doctor draws a blood sample from a patient's vein with a needle and wraps the injection site with a bandage as needed to prevent bleeding, explains MedlinePlus. The doctor mixes the sample with certain chemicals and waits to see the time period the sample takes to clot. For accurate results, the patient should inform the doctor of any medications he is taking beforehand because some medications, such as blood thinners, affect the results. This information helps the doctor advise the patient on the test preparation, which may include telling the patient to stop taking the medication temporarily.
Normal INR test results for people who don't take blood-thinning drugs range from 0.8 to 1.1, and 2.0 to 3.0 is the normal range for those who take such medications, reports MedlinePlus. Abnormal results may be due to conditions such as an incorrect dosage, bleeding disorders, low vitamin K and liver disease.