What is being tested?

Prothrombin is a protein produced by the liver. It is one of several proteins, known as clotting factors, that help your blood stay at the right consistency. The INR test measures this clotting factor by measuring how long it takes for your blood to clot.

Why would I need this test?

Blood-thinning medicines such as warfarin are used to prevent blood clots. They are usually prescribed for people who have atrial fibrillation (where the heart beats unevenly) or who have had artificial valves fitted. How well the blood thinner is working is measured by how it increases the blood clotting time. So if you are taking a blood thinner, you might need an INR test.  

Blood thinning medicines don’t actually make your blood thinner. They work by increasing the time it takes for your blood to clot.

Your doctor can use an INR test to make sure the dose and type of blood-thinning medicine is right for you. 

Blood clotting helps your body stop bleeding. But blood clots can also cause strokes or heart attacks. The INR test helps balance the risk of internal bleeding against the risk of blood clotting.

This test can also be used to:

• check for bleeding conditions
• check for blood clotting problems
• check how well your liver is working
• check your level of vitamin K (needed for blood clotting).

How to prepare for this test

No special preparation is needed. However many medicines, foods and illnesses can affect this test. So tell your doctor about any medicines and supplements you are taking, as well as any changes in your diet.

Understanding your results

When you are having warfarin treatment, your doctor will probably have a target INR for you. If your INR is too high, it might mean your blood clots too slowly, and there is a risk of you bleeding. If your INR is too low, it might mean you have a risk of developing a blood clot.

If you are not having blood-thinning treatment and your INR is higher than normal, you might need further tests.

More information

Lab Tests Online has more information about INR testing. You can also read more about blood tests in general.

About blood testing

Visit our ‘Guide to blood testing’ to learn more about blood tests in general with information such as:

• what to consider before having the test
• what happens during a blood test
• results accuracy
• blood tests cost.