PT

What is being analysed?
The prothrombin time (PT) measures the time it takes for a clot to form in a sample of blood or plasma. In the body, clotting normally involves a series of sequential reactions. One of the final steps is the conversion of prothrombin into thrombin. Prothrombin is one of the clotting factors made by the liver. The PT assesses how well these factors work and whether a clot can form in an appropriate time. The prothrombin time is generally measured in seconds and compared with the value obtained in healthy people. Normal values can vary because the reagents used to perform the test differ between laboratories and can change over time even within the same laboratory. A specific committee of the World Health Organization (WHO) introduced and recommended the International Normalised Ratio (INR) to standardise test results worldwide for people taking oral anticoagulants (e.g. Coumadin). The INR is a mathematical calculation that corrects for variations due to the different reagents used to perform the PT and makes results more comparable between laboratories. Most laboratories now report PT results both in seconds and as an INR. However, it is advisable not to use PT results expressed as an INR in patients who are not taking oral anticoagulants.

How is the sample collected for testing?
The sample is usually obtained by drawing blood from a vein in the arm.

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?
Because the test assesses the blood’s ability to clot, the PT in seconds and the INR (PT/INR) can be used to investigate bleeding or clotting problems. The most common use is monitoring the effectiveness of oral anticoagulant therapy such as warfarin (Coumadin). These anticoagulants reduce the formation of blood clots. They are prescribed long term for patients who have had recurrent episodes of abnormal clot formation, including patients who have had venous thromboembolism (deep vein thrombosis and/or pulmonary embolism), heart attack or stroke. Oral anticoagulant therapy may be used as a preventive measure in patients with atrial fibrillation, in patients who have had heart surgery, in people with mechanical or biological heart valves and, for short periods, in patients undergoing surgery, especially orthopaedic surgery (hip or knee replacement). Anticoagulants must be carefully monitored to maintain a balance between preventing clots and the risk of bleeding.

When is it requested?
If you are taking an oral anticoagulant, your doctor will regularly check your PT/INR to confirm that the prescription is effective and that your PT value is within the specific therapeutic range. There is no set frequency for the test. The doctor will order it as often as needed to make sure that the medicine is having the desired effect, i.e. prolonging clotting time to a therapeutic level without exposing the patient to an excessive risk of bleeding or bruising. The PT may be ordered for patients who are not taking oral anticoagulants to check for clotting factor deficiencies that predispose to bleeding, or to assess the ability to stop bleeding before surgery.

What does the test result mean?
Note: there is no single reference range for this test. Reference values can differ between laboratories because they depend on many factors, including the patient’s age, sex, the reference population and the analytical methods. Each laboratory’s report should give specific reference ranges for each analyte.

For the PT, test results depend on the method used; results are expressed in seconds or as an INR and compared with a reference range derived from healthy people. Most laboratories report PT results as an INR. Patients on oral anticoagulant therapy should keep their INR within the specific therapeutic range, which varies depending on the condition and on the patient.
The doctor uses the PT result (PT/INR) to adjust therapy so that the value falls within the specific therapeutic range. A prolonged prothrombin time means that the blood takes longer to form a clot. If you are not taking anticoagulants and your PT is prolonged, further tests may be needed to find the cause.

Is there anything else I should know?
Some substances, such as alcohol, can affect the PT/INR result. Some medicines, including antibiotics, can increase the PT/INR. Other medicines, including barbiturates, oral contraceptives, hormone replacement therapy and vitamin K (whether in multivitamin supplements or in food), can decrease the PT. Some foods (for example liver, broad-leaved vegetables, green tea, broccoli, chickpeas, cabbage, turnips, soya products and kiwi fruit) contain a lot of vitamin K and can therefore alter the effect of oral anticoagulants and so affect PT results. The doctor must be kept informed of any changes in medicines and diet in a patient on oral anticoagulant therapy, so that PT/INR results can be interpreted and used correctly.

1. Can I do the test at home?
2. Should I have the test at the same time of day?
3. My PT levels sometimes change, but my doctor does not change my treatment. Why?

1. Can I do the test at home?
Yes. Several devices for home PT/INR testing are now available. However, self-testing must be carried out as part of a home monitoring programme, which involves patient education and clearly defined treatment management protocols.

2. Should I have the test at the same time of day?
It is not generally necessary, although it is advisable, to measure the PT/INR at a set time of day. It is much more important to take the oral anticoagulant at the same time every day to keep the effect of the medicine constant. If your doctor increases or decreases the dose, the test may need to be rechecked within a few days to assess the effect of the change on the PT/INR (the effect is not immediate).

3. My PT levels sometimes change, but my doctor does not change my treatment. Why?
Many medicines can change the effect of oral anticoagulant therapy and PT/INR results, and so can the onset of some illnesses. Some foods, such as liver, green tea, broad-leaved vegetables, broccoli, chickpeas, cabbage, turnips, soya products and kiwi fruit, contain a lot of vitamin K and can change the effect of oral anticoagulant therapy and therefore the PT result. The sampling technique and any difficulty in obtaining the sample can also affect the result. If a patient’s PT/INR is unstable or does not respond to therapy as expected, more frequent PT/INR checks may be needed.