FaridabadBy Dr B.C. Gupta, MD · 2026-10-15
A prothrombin time test, usually reported together with the INR, measures how long it takes for a sample of blood to form a clot. It is one of the routine tests used to check how well the blood's clotting system is working.
What the test measures
Clotting depends on a set of proteins made by the liver, together with vitamin K. Prothrombin time measures how long the clotting proteins take to form a clot, and the result is reported in seconds. Different laboratories use slightly different reagents, so the prothrombin time measured in seconds is not directly comparable between laboratories. The INR, or international normalised ratio, solves this by expressing the result as a ratio against a standard, so a doctor anywhere can read the same number.
Why the INR is used to monitor blood thinners
The most common reason for checking an INR is to monitor treatment with a blood thinner such as warfarin. The dose needs to be high enough to prevent clots but not so high that bleeding becomes dangerous, and the INR is how that balance is checked. For most people on warfarin the target is usually between 2 and 3, though the exact target is set by the doctor and can differ depending on the reason for treatment.
What a high or low result can mean
A higher than expected INR means the blood is taking longer to clot, which can happen with a high dose of a blood thinner, with liver disease, or with a shortage of vitamin K. A lower than expected INR means the blood is clotting more quickly, which can mean the blood thinner dose is too low to protect against clots. Neither direction is interpreted on its own; the result is read alongside your treatment and your symptoms.
How to prepare
A blood sample is drawn from a vein in the arm. Fasting is not needed for this test. If you take a blood thinner, take it as your doctor has instructed unless told otherwise, and tell the centre which medicine you take and when you last took it, because that affects how the result is read. Results are usually available the same day.
When it is ordered
Prothrombin time and INR are checked when someone starts or continues a blood thinner, before surgery, when there is unexplained bleeding or bruising, and when the liver is being assessed, because the liver makes the proteins this test measures. The centre performs PT/INR along with APTT and other clotting tests, so the clotting system can be assessed from a single sample.
This article is for general information only and is not medical advice. Always discuss your results and any health concerns with your doctor.
