Prothrombin Time (PT with INR)

Specimen Required

Sodium Citrate (PT Vial)

Fasting Required

Purpose of the Test

To help diagnose a bleeding disorder; to check clotting efficiency prior to a surgical procedure; 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 this test is required

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 the Test Detects

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.See MoreSee Less

Preparation for the Test

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.

Sample Requirements

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.

Additional Notes

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.

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