Prothrombin Time (PT) with International Normalized Ratio (INR) – Plasma

PT stands for prothrombin time, a blood test that measures how long it takes your blood to clot. INR, or international normalized ratio, is a standardized way of reporting that result so it means the same thing in every laboratory. Together, PT/INR show whether your blood's clotting system is working normally.

PT measures the time it takes for your blood to clot after an anticoagulant (like tissue factor) is added. INR standardizes PT results to compare across labs, accounting for differences in reagents. Together, they reflect the activity of clotting factors in your blood.

Doctors order this test to check for bleeding or clotting problems, to monitor blood thinners such as warfarin, to evaluate how well your liver is working, or to look into easy bruising and unusual bleeding. If you take an anticoagulant, regular testing helps your provider keep your dose in a safe and effective range.

Your doctor may order PT/INR to evaluate bleeding risks, monitor warfarin or other anticoagulant therapy, diagnose clotting disorders (e.g., hemophilia), or check liver function (since clotting factors are made in the liver).

A health professional draws a small sample of blood from a vein in your arm, usually on the inside of the elbow. The needle stick takes only a few moments, and the sample is sent to a laboratory for analysis. If you take warfarin, some clinics may use a simple finger-stick device instead of a vein draw.

A small blood sample is drawn from a vein. The blood is mixed with a specific activator and measured for clotting time. Plasma is used because it contains the clotting factors without platelets.

For people who are not taking blood thinners, a normal PT usually falls between about 11 and 13.5 seconds, with an INR close to 1.0 to 1.1. Most patients on warfarin aim for an INR between 2.0 and 3.0, although your personal target may differ based on your condition. Your provider will interpret your numbers for you.

Results are expressed as PT time (in seconds) and INR (a ratio). Normal INR is typically 0.8–1.2, but ranges vary by condition and treatment. Your doctor will interpret results based on your clinical needs.