Finding the right tests…
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Finding the right tests…
Serology
PT/PC/INR combines three ways of reporting one blood test: prothrombin time in seconds, prothrombin concentration as a percentage of normal, and INR, a standardised ratio that lets results be compared across different labs and machines. All three come from timing how long your plasma takes to clot once a specific reagent is added. Doctors order this most often to monitor warfarin, a blood-thinning medicine, since warfarin works by slowing exactly the clotting pathway this test measures. It is also used to check liver function, since the liver makes most of the clotting factors this test depends on, and to investigate unexplained bleeding.
PT is reported in seconds and compared against the lab's own reference range; INR takes that same result and adjusts it mathematically so it means the same thing whichever lab or machine produced it. On warfarin, your doctor sets a target INR range for your specific situation and adjusts your dose to keep you inside it, since running too low raises clot risk and too high raises bleeding risk. Prothrombin concentration expresses the same underlying clotting activity as a percentage of normal rather than a ratio, and some lab reports show it alongside the INR.
PT is the raw time, in seconds, that a sample of your plasma takes to clot. INR takes that raw time and adjusts it using a factor specific to the reagent the lab used, so a result from one lab means the same thing as a result from another. Prothrombin concentration expresses the same underlying clotting activity as a percentage compared to normal plasma - a different way of presenting the same measurement, still used on some lab reports alongside the INR.
Warfarin blocks vitamin K, which your liver needs to make several clotting factors, and PT/INR is sensitive to exactly those factors. Someone newly started on warfarin has this test checked often until their dose is stable, then at longer intervals afterward. A result outside the target range usually means a dose adjustment, not stopping the medicine altogether - that decision belongs to the prescribing doctor.
- Assessing liver function, since a sick liver cannot make normal amounts of clotting factors - Investigating unexplained bruising or bleeding - Screening before some surgeries - Checking for vitamin K deficiency
An INR outside the expected range needs your doctor's interpretation alongside your medicines, diet, and any liver history - it is not read as a stand-alone verdict.
For someone not on a blood thinner, a normal INR is typically about 0.8 to 1.1, close to a PT of 11 to 13.5 seconds. Ranges vary slightly by lab and reagent, so check the reference interval on your own report.
Many people on warfarin are kept in a target range of roughly 2.0 to 3.0, though the exact target depends on why warfarin was prescribed. Ranges vary by lab and by the condition being treated, so ask your doctor what target applies to you.
Prothrombin concentration reports the same underlying clotting measurement as a percentage of normal activity, rather than as the INR's standardised ratio. Some lab reports show both figures for the same blood draw.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
The liver produces most of the clotting factors that PT and INR measure, so liver disease that impairs this production shows up as a prolonged PT and a raised INR, even without any blood thinner involved.
There's no fixed fasting requirement for this test. Ask your doctor or the centre when you book, particularly if other blood work is planned at the same visit.
Vitamin K intake and several medicines can shift INR results in someone on warfarin, which is why doctors ask about diet and other prescriptions when reviewing a result. Ask your doctor before changing your diet or medicines while on warfarin.