Also Known As
Prothrombin Time
Formal Name
Prothrombin Time
This article was last reviewed on
This article waslast modified on 26 October 2018.
At a Glance
Why Get Tested?

To help diagnose a bleeding disorder; to help estimate the severity of liver disease. A tightly controlled version of the PT called the International Normalised Ratio (INR) is used to measure the effect of anticoagulant drugs such as warfarin.

When To Get Tested?

No test preparation is needed. If the patient is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken. It is essential the blood is taken quickly and smoothly and that the anticoagulant bottle is filled exactly to the designated mark or the result may be inaccurate.

Sample Required?

A blood sample taken from a vein in the arm

Test Preparation Needed?

None needed, although if you are receiving anticoagulant therapy, the sample should be collected before taking your daily dose.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

The test measures how long it takes for your blood to begin to form clots. Prothrombin is a plasma protein produced by the liver. Clotting is caused by a series of clotting factors which activate each other, including the conversion of prothrombin to thrombin. The test used to measure the activity of this clotting factor is called the prothrombin time, or PT.

How is the sample collected for testing?

Blood is collected by needle from a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. If the patient is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken. It is essential the blood is taken quickly and smoothly and that the anticoagulant bottle is filled exactly to the designated mark or the result may be inaccurate.

Accordion Title
Common Questions
  • How is it used?

    The Prothrombin time (PT) test, standardised as the INR test is most often used to check how well anticoagulant tablets such as warfarin and phenindione are working. Anticoagulant tablets help prevent the formation of blood clots (they do not "thin the blood" as is commonly thought). This is particularly important in people with heart conditions such as atrial fibrillation, with artificial heart valves and for people with blood clots. The effect of drugs such as warfarin can be determined by the prolongation of the PT (measured in seconds), or increase in the INR (a standardised ratio of the patient's PT versus a normal sample), and the dose adjusted accordingly.

    The test can also be used to diagnose a bleeding disorder; a doctor will compare the PT with other clotting tests to indicate where in the clotting system a defect might lie.

    Lastly, when liver disease becomes serious, the liver loses the ability to make essential proteins including clotting factors. The PT is one of the more sensitive tests to monitor this.

  • When is it requested?

    If you are taking an anticoagulant drug, your doctor will check your INR regularly to make sure that your prescription is working properly and that the INR is appropriately increased. There is no set frequency for doing the test. Your doctor will request it often enough to make sure that the drug dose is correct. Occasionally the PT or INR may be used on a patient who is not taking anti-coagulant drugs—to check for a bleeding disorder, liver disease or vitamin K deficiency, or to ensure clotting ability before surgery.

  • What does the test result mean?

    The test result for PT depends on the method used; results will be measured in seconds.
    Most laboratories report PT results that have been adjusted to the International Normalised Ratio (INR). Patients on anticoagulant drugs usually have a target INR of 2.0 to 3.0 (i.e. a prothrombin time 2 to 3 times as long as in a normal patient, using standardised conditions). For some patients who have a high risk of clot formation, the INR needs to be higher: about 3.0 to 4.0. Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you. An increased Prothrombin time or INR means that your blood is taking longer to form a clot. If you are not taking anticoagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause.

    Interpretation of PT and PTT in Patients with a Bleeding or Clotting Syndrome

    The PT is often performed along with another clotting test called the PTT (or sometimes the APTT or KCCT). Comparison of the two results can give your doctor information as to the cause of a bleeding problem.

    PT result ptt result Possible conditions present
    Prolonged Normal Liver disease, decreased vitamin K, decreased or defective factor VII
    Normal Prolonged Decreased or defective factor VIII, IX, XI or XII, von Willebrand disease, or lupus anticoagulant present
    Prolonged Prolonged Decreased or defective factor I, II, V or X, liver disease, disseminated intravascular coagulation (DIC)
    Normal Normal  Decreased platelet function, thrombocytopenia, factor XIII deficiency, mild deficiencies in other factors, mild form of von Willebrand’s disease, weak collagen
  • Is there anything else I should know?

    Some substances you consume - various foods, alcohol and many medications - can interfere with the PT test. Antibiotics and painkillers can increase PT and INR. Oral contraceptives, hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT and INR. Make sure that your doctor knows all the drugs you are taking and any changes in medication so that the PT results are interpreted correctly.

  • INR versus PT: which test should I have?

    The PT and INR is in essence the same test. The substances used to do the test vary somewhat and a patient’s PT may vary slightly from laboratory to laboratory.

    The INR is a highly-controlled version of the PT using standardised ingredients and the results are exactly reproducible no matter which laboratory or in which country the test is performed. The INR is specifically used to measure the exact effect of warfarin in the blood. It can also measure the effects of vitamin K deficiency (warfarin works by inactivating vitamin K and hence the activity of several vitamin K-dependent clotting factors in the blood). By standardising the INR, a medical professional can adjust the dose of warfarin to give the appropriate degree of anticoagulation. The higher the INR the less likely is a clot, but the more likely a bleed. Many patients have a target INR of 2.0 – 3.0 as an ideal compromise reducing the chances of a clot while being safe with respect to bleeding. The target range may be lower or higher than this depending on individual circumstances.

    The INR is a frequently performed test which is commonly quoted alongside a PT in a laboratory report. In some respects this is the same result but strictly the value of the INR is only in the accuracy with which it can measure the effect of warfarin, or severity of vitamin K deficiency.

  • Can I do this test at home?

    Yes, although it may be harder to guarantee the accuracy of tests performed at home. Ask your doctor whether home testing is appropriate for you.

  • Should I have it done at the same time of day?

    Only in the induction period, when a "loading" dose is frequently given and correctly timed INRs are important for predicting the long-term "maintenance" dose. You should ideally take your anti-coagulant tablets in the evening however. This allows your doctor to modify your dose up or down on the same day as your INR test.

  • My PT/INR results vary sometimes, yet my doctor doesn’t change my prescription. Why?

    The use of any of the drugs mentioned above can alter your results, as can the use of diuretics and antihistamines and the onset of illness or allergies. Certain foods, such as beef and pork liver, green tea, cabbage, kale, broccoli, turnip greens, chickpeas and soya bean products contain large amounts of vitamin K and can alter PT results. The blood collection technique and the difficulty in obtaining the blood sample can also affect test results.

    It is not ideal to change the warfarin dose rapidly or very frequently (more than every 5 days), except when finding a starting dose. Warfarin has a complicated effect on several coagulation factors and some coagulation factors in the blood. Their activities fall at differing rates and it is only in the steady state (after about 5 days of the same dose of warfarin) that the INR truly reflects a standardised state of clotting. If your control is generally very good but an occasional result is just outside the desired INR range on one occasion, the medical professional may decide it is better to stay on a generally effective dose with which you are familiar, only changing it if there is a significant or persistent change in the INR.