The health care provider uses a needle to take blood from one of your veins. The blood collects into an airtight container. You may be given a bandage to stop any bleeding. If you are taking a medicine called heparin, you will be watched for signs of bleeding.
The lab specialist will add chemicals to the blood sample and see how long it takes for the plasma to clot.
How to prepare for the test
Make sure your doctor knows about all the medicines you are taking. This including things you buy without a prescription such as over-the-counter medicines, herbs, and supplements. Your doctor may tell you to stop taking certain drugs before the test. For example, blood thinners can affect the results of this test.
Do not stop taking any medicine without first talking to your doctor.
How the test will feel
You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.
Why the test is performed
The most common reason to perform this test is to monitor your levels when you are taking a blood-thinning medicine called warfarin.
You are likely taking this medicine to prevent blood clots
Your health care provider will check your PT, or prothrombin time, regularly.
If you are taking warfarin to prevent clots your doctor will most likely choose to keep your INR between 2.0 and 3.0.
Depending on why you are taking the blood thinner, the desired level may be different.
Even when your INR stays between 2.0 and 3.0, you're more likely to have bleeding problems.
INR results higher than 3.0 may put you at even higher risk risk for bleeding.
INR results lower than 2.0 may put you at risk for developing a blood clot.
A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:
The wrong dose of medicine
Taking certain over-the-counter medicines, vitamins, supplements, cold medicines, antibiotics, or other drugs
Eating a food that changes the way the blood-thinning medication works in your body
Your healthcare provider will teach you about taking warfarin (Coumadin) the proper way.
What the risks are
This test is often done on people who may have bleeding problems. The risks of bleeding and hematoma in these patients are slightly greater than for people without bleeding problems.
Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 174.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.