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Prothrombin Time Test and INR (PT/INR)
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Prothrombin Time Test and INR (PT/INR)

A PT/INR test measures how quickly your blood clots, helping providers monitor blood-thinning medications and diagnose bleeding or clotting disorders.

What is a prothrombin time test with an INR (PT/INR)?

A prothrombin time (PT) test measures how many seconds it takes for a clot to form in a blood sample. Because results can vary between labs or methods, results are often converted into an INR (international normalized ratio) to allow for consistent comparison.

Prothrombin is a protein produced by the liver and is one of several clotting (coagulation) factors. When you are injured, these factors work together to form a clot, which prevents excessive blood loss. If levels are too low, you may bleed too much; if levels are too high, you are at risk for dangerous clots in your veins or arteries.

Other names for this test include: prothrombin time/international normalized ratio, PT, and pro time.

What is it used for?

The PT/INR test is primarily used to:

  • Monitor the effectiveness of warfarin (an anti-coagulant/blood-thinner, such as Coumadin).
  • Identify the cause of abnormal blood clots or unusual bleeding.
  • Assess clotting ability prior to surgery.
  • Check for liver problems.
  • Check for vitamin K deficiency.

It is often performed alongside a partial thromboplastin time (PTT) test, which evaluates different coagulation factors.

Why do I need a PT/INR test?

If you take warfarin, you need regular testing to ensure your dosage is correct. If you are not on blood-thinning medication, you may need the test if you exhibit symptoms of a disorder.

Symptoms of a bleeding disorder include:

  • Unexplained heavy bleeding
  • Easy bruising
  • Unusually heavy nose bleeds
  • Unusually heavy menstrual periods

Symptoms of a clotting disorder include:

  • Leg pain, swelling, or tenderness
  • Redness or darkening of the skin on the leg
  • Skin that feels warm
  • Trouble breathing or cough
  • Chest pain
  • Rapid heartbeat and breathing

What happens during a PT/INR test?

The test requires a blood sample taken from a vein or a fingertip.

  • Vein sample: A professional collects blood from a vein in your arm using a small needle.
  • Fingertip sample: Often used for at-home monitoring, this involves using a small needle to puncture the fingertip and placing a drop of blood onto a test strip or device.

What do the results mean?

Results are typically reported as INR levels for those on warfarin or as a PT time (in seconds) for others.

  • INR too low: Indicates blood clots too easily, increasing the risk of dangerous blood clots.
  • INR too high: Indicates blood clots too slowly, increasing the risk of dangerous bleeding.

Factors that can affect your results include alcohol consumption, high-fat diets, soybean/canola oils, and foods high in vitamin K (such as green leafy vegetables). Always inform your provider of any medications or supplements you are taking.

Frequently Asked Questions

You must maintain a consistent intake of vitamin K, as sudden changes can drastically alter your INR. Avoid large fluctuations in leafy green vegetables like spinach, kale, and broccoli, as well as supplements containing high doses of vitamin K. It is also critical to limit alcohol consumption and avoid herbal supplements such as ginkgo biloba, garlic, and ginseng, which can interfere with warfarin's effectiveness and increase bleeding risk. How quickly does my INR change after I adjust my warfarin dose, and when should I get retested?::Warfarin has a long half-life, so changes in dosage typically take three to five days to fully affect your INR. If your provider adjusts your dose, you will usually need a follow-up blood test within that window to ensure your INR is within the therapeutic range. Do not skip scheduled appointments, as missing a test can lead to dangerous levels of anticoagulation or clotting. Can common over-the-counter pain relievers like ibuprofen or naproxen affect my PT/INR results?::While NSAIDs like ibuprofen and naproxen do not directly change the PT/INR number, they significantly increase your risk of bleeding by affecting platelet function and irritating the stomach lining. If you are on warfarin, using these medications can lead to serious gastrointestinal bleeding. Acetaminophen is generally safer for pain relief, but high doses can also affect INR, so always consult your provider before taking any new medication. What should I do if I miss a dose of warfarin or take an extra one by accident?::If you miss a dose, take it as soon as you remember on the same day, but do not double up on the next day. If you take an extra dose, contact your healthcare provider immediately, as this can cause your INR to spike dangerously high. Keep a log of your doses and any unusual symptoms like dark stools or excessive bruising to share with your provider during your next appointment.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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