INR (prothrombin time): what it is, what it tells you and when it’s tested

INR shows how long your blood takes to clot, compared with a healthy person. It comes from the prothrombin time (PT), written so it can be compared between labs. It matters most to people who take anticoagulants such as warfarin, and to doctors in hospital. If you have bleeding you cannot stop, call 112 or go to the emergency department.

When it’s tested

It is mostly used to check anticoagulants such as warfarin and acenocoumarol. People who take them have an INR at regular intervals, more often at the start, and their doctor or anticoagulation clinic adjusts the dose.

Doctors also order it if you bleed easily, if they have doubts about your liver, before some operations or in the emergency department.

In people who feel well and take no anticoagulant, it is almost always normal. That is why it is not in the Rosa panel. Instead Rosa measures the liver tests (your liver makes the clotting factors): liver enzymes, bilirubin and albumin, along with platelets.

What it tells you, and what it does not

The higher the INR, the longer your blood takes to clot. Without anticoagulants it is usually around 1.

If you take warfarin, the INR shows whether the dose is right for you: not too little, with a risk of clots, and not too much, with a risk of bleeding.

INR is not used for the newer anticoagulants, such as apixaban, rivaroxaban, edoxaban and dabigatran: their dose is not set by the INR.

That is why it is read together with other tests:

Normal INR levels

Without anticoagulants, many lab reports give 0.8 to 1.2. Your own report may print slightly different limits.

If you take warfarin or acenocoumarol, the right value is different. For most people the target is between 2 and 3. For some mechanical heart valves it is higher.

Your target is set by your doctor or anticoagulation clinic, based on why you take the medicine. It is written in your treatment plan.

If your INR is high or low

Why INR can be high

If you take warfarin, that is the most common cause. Without anticoagulants:

If you take warfarin

INR changes with food, medicines and your health. Antibiotics, some painkillers, some supplements and sudden changes in diet can push it up or down.

You do not need to avoid leafy greens: what matters is eating them regularly, without big changes from one week to the next. Before starting a new medicine or supplement, tell your doctor.

If your INR is outside your target, do not change your dose yourself and do not skip it. Call your doctor or anticoagulation clinic the same day: they tell you what to do.

What can skew the result

A few things change the value without a real clotting problem.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal INR levels? Without anticoagulants, many lab reports give 0.8 to 1.2. If you take warfarin, the target is usually between 2 and 3, but your doctor sets it.

What is the difference between PT and INR? PT is the clotting time in seconds, and it varies from lab to lab. INR is the same result made comparable everywhere. That is why INR is used to follow warfarin.

High INR: what should I do? If you take warfarin, call your doctor or anticoagulation clinic the same day and do not change your dose yourself. If you do not, show the report to your doctor. If you are bleeding heavily, call 112.

What can I not eat on warfarin? You can eat almost everything. Regularity is what counts: leafy greens are fine, but without big changes from one week to the next. Before supplements or new medicines, ask your doctor.

If I take apixaban or rivaroxaban, do I need an INR? Usually not. These anticoagulants are not adjusted with the INR. Your doctor decides which checks you need.

Does Rosa test INR? No. Rosa measures the liver tests, such as liver enzymes, bilirubin and albumin, and platelets. If you need an INR, your doctor or anticoagulation clinic orders it.

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