• The Prothrombin time (PT) and International Normalised Ratio (INR) test measures the time it takes for your blood to clot.
• It is used to monitor the effects of the anticoagulant medication, warfarin to make sure you are taking the right dosage.
• It is also used to diagnose bleeding disorders or make sure your blood is clotting properly before you have surgery.
• The results of the PT test vary between laboratories. This is because different equipment and test methods are used. The International Normalised Ratio (INR) was developed to standardise PT results. It is a calculation based on the PT result.
• Using the INR makes it easier to compare and interpret test results.

Prothrombin is a protein made by your liver that plays a key role in blood clotting. It is one of several clotting factors your body uses to stop bleeding. If you do not have enough prothrombin, or if it is not functioning properly, it takes longer for your blood to clot.

Clotting is a normal part of healing. When you have an injury and it starts to bleed, your body triggers a sequence of chemical reactions to create a blood clot and stop the bleeding. This is the coagulation cascade.

Substances called clotting factors are activated one after the other, then threads of a protein called fibrin are produced.

The fibrin threads are glued together to form a fibrin net that catches platelets and helps hold them together to form the clot.

Platelets are tiny plate-shaped cells that circulate in your blood. They bind together to form clots. They transform in shape and become very sticky to begin the formation of a blood clot.

Once a clot is formed, other substances are activated to slow the clotting process. They eventually begin to dissolve the clot so that the clot is removed when the injury is healed.

There must be enough of each clotting factor, and each must function properly, in order for normal clotting to occur.

One of these clotting factors is prothrombin. One of the steps in the clotting cascade is the conversion of prothrombin to thrombin.

Prothrombin tests and the International Normalised Ratio (INR)

The Prothrombin Time (PT) test measures the time in seconds it takes for prothrombin to convert to thrombin. This is the actual clotting time. An increased prothrombin time means your blood is taking longer to clot.

Variation in the chemicals and instruments used by different laboratories to perform the PT test means the results they give can vary even when the actual clotting time is the same. An alternative way to show this result is as the International Normalized Ratio or INR which is a standardized value that helps compare PT results across different laboratories.

Your doctor may request a prothrombin test for two major reasons:

• If you are taking the drug warfarin they will want to check if your dose is at the optimal level. They will request the INR form of the PT test.
• If you have symptoms or a clinical history of a clotting disorder they will request a PT and possibly other tests.

Management of warfarin therapy

If you are regularly taking warfarin, you will need an INR to make sure you are taking the right dose. Your doctor will check your INR regularly to make sure that your dose is within the target range.

Having the right dose of warfarin is important. It needs to achieve just the right balance – too much and there is a risk of bleeding, too little and there is more chance of clots. There is no set frequency for doing the test. Your doctor will order it often enough to make sure that your dosage is correct.

Investigation of a bleeding disorder

If your PT result is described as prolonged it means your blood is taking longer to clot. Further investigation is required especially if you have unusual bleeding. One of these tests is the activated partial thromboplastin time or aPTT. The results of both PT and aPTT tests together can help make a diagnosis or guide further testing to determine the cause of abnormal results including:

• Liver disease
• Vitamin K deficiency
• Lack of key clotting factors
• Other anticoagulation medication including Non-Vitamin K Antagonist Oral Anticoagulants (NOACs)

For more information about other tests that may be required please see Coagulation Profile and Coagulation Factors.

Sample

Blood.

Any preparation?

Some common medications can interfere with the prothrombin time PT test and give a misleading result. Antibiotics, aspirin and cimetidine can increase prothrombin time. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease prothrombin time. Drinking alcohol can also affect prothrombin time results as can foods that contain high levels of vitamin K.

Make sure that your doctor knows all the medications you are taking so that the prothrombin time results are interpreted correctly.

Reading your test report
Your results will generally be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

Management of warfarin therapy

Most people on warfarin have a target INR range of 2.0 to 3.0.

For some people who have a higher risk of clotting, the INR needs to be higher. Your doctor will use your INR to adjust the dose of your warfarin to get the INR into the target range that is right for you.

There are fewer people using warfarin now due to the introduction of more modern anticoagulant drugs such as dabigatran, rivaroxaban and apixaban. One of the advantages of these drugs is they do not require monitoring of INR values. However, they can cause an abnormal PT/INR result if these tests are performed whilst on these medications.

Investigation of a clotting disorder

Interpretation of PT and APTT results and what they mean for different bleeding disorders is shown in the table.

Reference Intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people.
• When compared against them your results may be flagged high or low if they sit outside the range.
• Reference ranges may vary between different age groups.
• Reference intervals may vary between labs so only those results that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. Your doctor needs to take your medical history to help interpret your result.

Another reason to measure your PT/INR is prior to you having surgery particularly if you have a clinical history that suggests abnormal clotting.

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements, as these may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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