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.
Prothrombin Time and International Normalised Ratio (PT/INR)
The PT is commonly requested, along with the APTT, as part of a routine coagulation screen. A routine coagulation screen may be performed if a patient has unexplained bleeding or easy bruising, or prior to surgery to assess efficiency of a patient’s clotting system.
Patients on warfarin should have their INR measured regularly.
A blood sample taken from a vein in the arm.
In some cases, a lancing device may be used to obtain a drop of blood from the fingertip, to measure the INR.
No test preparation is needed. If the patient is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken. The blood sample should be collected into a sodium citrate blood tube to avoid activation of clot formation prior to testing. 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.
The Prothrombin Time, or PT, measures the time taken (in seconds) for the blood to clot following addition of an activating enzyme known as thromboplastin. The clotting cascade activated is dependent on the activity of specific clotting factors found in the blood, namely, factor II (prothrombin), V, VII, X and fibrinogen. Therefore a prolonged PT may indicate deficiencies or inhibitors of one or more of these proteins.
How is it used?
The Prothrombin time (PT) test can be used to diagnose a bleeding disorder; a healthcare professional will compare the PT with other clotting tests to indicate where in the clotting system a defect might lie (see table in ‘What does the test result mean?’ section).
The PT may also be measured before a surgical procedure (along with the APTT and fibrinogen) to check there are no defects in the clotting system that may lead to unexpected bleeding complications. The PT is one of the more sensitive tests to monitor serious liver disease (when the liver loses the ability to make essential proteins including clotting factors).
One of the most common uses of the PT is to monitor the International Normalised Ratio (INR) of patients 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.
When is it requested?
If you are taking an anticoagulant drug, your healthcare team 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 team 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. 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.
For monitoring of vitamin K-antagonists, such as warfarin, PT results are adjusted to the International Normalised Ratio (INR). People 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 person not on warfarin with normal clotting, using standardised conditions). For some people who have a high risk of clot formation, the INR needs to be higher: about 3.0 to 4.0. Your healthcare team will use the INR to adjust your drug to get the PT into the range that is right for you.
Interpretation of PT and aPTT in Patients with a Bleeding or Clotting Syndrome
The PT is often performed along with another clotting test called the aPTT (or sometimes the PTT or KCCT). Comparison of the two results can give your healthcare team information as to the cause of a bleeding problem.
PT result aPTT 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. Vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT and INR. Make sure that your healthcare team knows all the drugs you are taking and any changes in medication or lifestyle 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 person’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 healthcare professional can adjust the dose of warfarin to give the appropriate degree of anticoagulation. The higher the INR the less likely a person is to clot, but the more likely they are to bleed. Many patients have a target INR of 2.0 – 3.0, considered an ideal compromise between reducing the chances of clot formation whilst 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?
Point of care testing (POCT) can be used to measure the INR in the clinic or at home instead of in a lab. This is done using a drop of blood from your fingertip instead of a venous blood sample, and can give an immediate result. Although it is more common for POCT to be performed in the clinic or pharmacy, some patients can self-monitor at home. However, 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 healthcare team 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?
There are many factors that may affect the PT/INR results of a patient on warfarin. The use of any of the drugs mentioned above can alter your results, as can the onset of illness, changes in diet or weight loss or gain. 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 if consumed in excess. Drinking large amounts of alcohol, or cranberry juice or grapefruit juice can have affect the PT/INR. A missed or forgotten dose of warfarin may also be responsible for variable 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 in the blood, and their activities fall at differing rates. 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.