INR

What is being tested?
The prothrombin time (PT) measures how long it takes for a clot to form in a blood sample. In the body, clotting involves a series of sequential chemical reactions. One of the last steps is the conversion of prothrombin into thrombin. Prothrombin is one of several clotting proteins produced by the liver. The PT test assesses how well these clotting factors work and the body’s ability to form a clot within a certain amount of time. The PT is usually measured in seconds and compared with the values obtained in healthy people. Since the chemical reagents used for the test vary from one laboratory to another, and over time even in the same laboratory, normal values can also vary. To standardise results worldwide, a WHO (World Health Organization) committee developed and recommended the use of the international normalised ratio (INR) with the PT test for patients taking the anticoagulant warfarin (Coumadin). The INR is a calculation that removes the differences between the reagents used for the PT test and allows results from different laboratories to be compared. Most laboratories now report both the PT and the INR whenever a PT test is done. However, the INR should not be used to assess the PT in patients who are not on anticoagulants

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
Since the PT and INR (PT/INR) measure the blood’s ability to clot properly, they can be used to assess the tendency both to bleed and to form clots. A common use is to monitor the effectiveness of anticoagulant medicines such as warfarin (Coumadin). These anticoagulants help inhibit clot formation. They are prescribed long term for patients who have had recurrent inappropriate clots. These include patients who have had heart attacks, strokes and deep vein thrombosis (DVT). Anticoagulant treatment may also be given as a preventive measure to patients with artificial heart valves, and short term to patients having surgery such as knee operations. Anticoagulant medicines must be monitored continuously to maintain a balance between preventing clots and causing excessive bleeding.

When is it requested?
If you are taking an anticoagulant medicine, your doctor will measure your PT/INR regularly to make sure the treatment is working properly and that your PT is suitably prolonged. There is no set frequency for the test. Your doctor will request it often enough to monitor the effectiveness of the medicines, i.e. to increase the clotting time to a therapeutic level without causing bleeding or bruising. The PT test may be requested for patients who are not taking anticoagulants to investigate a bleeding disorder or to assess clotting ability before surgery.

What does the test result mean?
The PT result depends on the method used; results are expressed in seconds and compared with the average value obtained in the healthy population. The laboratory reports standardised PT results (INR). Patients on anticoagulants should have an INR of 2.0 to 3.0 with basic anticoagulant treatment. For patients at high risk of clot formation, the INR should be higher, about 2.5 to 3.5. Your doctor will use the INR to adjust the dose of the medicine so that your PT is in the best range for you. A prolonged prothrombin time means that your blood is taking too long to form a clot. If you are not taking anticoagulants and your PT is prolonged, further tests are needed to find the cause.

Is there anything else I should know?
Some substances you consume, such as alcohol, can affect the PT/INR result. Antibiotics, aspirin and cimetidine can increase the PT/INR. Barbiturates, oral contraceptives, hormone replacement therapy (HRT) and vitamin K, whether in a multivitamin supplement or a liquid nutritional product, can decrease the PT. Some foods (such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnips and soya) contain large amounts of vitamin K and can alter the PT result. Make sure your doctor knows about all the medicines you take and whether you have recently eaten any of these foods, so that the PT/INR result can be interpreted correctly

1. Do I have to have it at the same time of day?
2. My PT results sometimes vary, but my doctor does not change the medicines prescribed. Why?

1. Do I have to have it at the same time of day?
It is not generally necessary to measure the PT/INR at a particular time of day. However, it is important to take the anticoagulant medicine at the same time every day to maintain a steady level. If your doctor increases or decreases a dose, they may want to measure your PT again a few days later to judge the effect of the change on your PT/INR (the effect is not immediate).

2. My PT results sometimes vary, but my doctor does not change the medicines prescribed. Why?
Using one of the medicines mentioned above can change the result, as can the use of diuretics and antihistamines and the onset of an illness or allergy. Some foods, such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnips and soya, contain large amounts of vitamin K and can alter the PT result. The blood collection technique and difficulty in taking the sample can also affect the result. If your doctor is concerned about the stability of your PT/INR, they may want to test your blood more often