INR Test Explained: What Your Number Means if You Take Warfarin

INR (international normalised ratio) shows how long blood takes to clot compared with normal. In people not taking blood thinners, INR is usually around 1.0.

Common warfarin target ranges

Many people: INR 2.0 to 3.0
Some people, for example with certain mechanical heart valves: higher targets, such as 2.5 to 3.5
Your target is set by your clinician

What can change your INR

Antibiotics and many other medicines
Changes in diet, especially vitamin K-rich foods such as leafy greens
Alcohol
Illness, vomiting or diarrhoea
Missed or extra doses

Warning signs to act on

Unusual bruising or bleeding gums
Blood in urine or black, tarry stools
Severe headache or a fall with a head injury
Coughing or vomiting blood

Lab ranges vary

Reference ranges differ between laboratories and countries, and some use different units. Always compare your result with the range printed on your report and discuss it with your healthcare provider.

Why people take warfarin

Warfarin is an anticoagulant, a medicine that makes blood take longer to clot. It is used to prevent strokes in people with atrial fibrillation, to treat and prevent blood clots in the legs and lungs, and to prevent clots forming on mechanical heart valves.

Many people now take newer anticoagulants (DOACs) instead, but warfarin remains essential for some groups, including people with mechanical heart valves, certain types of valve disease, severe kidney disease or antiphospholipid syndrome, and in places where newer medicines are less available or affordable.

What happens at an INR check

An INR is measured either from a blood sample taken from a vein or from a finger-prick test using a portable machine. Many anticoagulation clinics and community pharmacies use finger-prick testing, and some people test at home after training.

Based on your result, your clinic will tell you whether to keep your dose the same, increase it or reduce it, and when to have your next test. When you first start warfarin or after a dose change, tests are frequent. Once your INR is stable, they may be needed only every few weeks.

Time in therapeutic range

Healthcare providers often look at “time in therapeutic range”, the percentage of time your INR stays within your target. The higher it is, the better protected you are from both clots and bleeding. If your INR is often outside your range, your clinic may look for causes or discuss switching to a different anticoagulant if that is suitable for you.

Food, drink and warfarin

Vitamin K, found in green leafy vegetables such as spinach, kale and broccoli, reduces warfarin’s effect. You do not need to avoid these foods, but keep your intake roughly the same each week
Cranberry juice and grapefruit products may increase warfarin’s effect; check with your healthcare provider
Binge drinking can raise your INR; moderate, steady drinking is safer if you drink at all
Some herbal remedies, such as St John’s wort, ginkgo and high-dose fish oils, can interact with warfarin

Medicines that commonly affect INR

Many antibiotics, including some commonly prescribed for chest and urine infections
Some antifungal medicines
Amiodarone and some other heart medicines
Anti-inflammatory painkillers, which also increase bleeding risk
Some epilepsy medicines
Always tell your anticoagulation clinic when any medicine is started, stopped or changed

Before procedures and dental work

If you need surgery, a dental extraction or another procedure, tell the team that you take warfarin well in advance. For many minor procedures, warfarin can be continued if your INR is in range. For others, it may need to be paused, sometimes with a short-term injection to protect you. Never stop warfarin on your own.

Carry an anticoagulant alert card

Carry an anticoagulant alert card or wear medical ID. In an emergency, it tells healthcare providers that you take warfarin, which affects how they treat bleeding and injuries.

What to do if your INR is too high

If your INR is above your target, your clinic will usually reduce your dose or ask you to miss a dose, and arrange an earlier test. If it is very high, or if you have bleeding, you may be given vitamin K to reverse warfarin’s effect.

Seek urgent medical help if your INR is high and you have any signs of bleeding, a severe headache, a fall or head injury, or you feel unwell. Bleeding inside the head can develop slowly after a fall, so do not ignore a head injury even if you feel fine at first.

What to do if your INR is too low

If your INR is below your target, you are less protected against clots. Your clinic may increase your dose and test again soon. Think about whether anything has changed, such as missed doses, more leafy greens than usual, a new medicine or a supplement, and tell your clinic. Seek urgent help for signs of a stroke (face drooping, arm weakness, speech problems) or a clot (painful swollen leg, sudden breathlessness, chest pain).

Questions to ask your anticoagulation clinic

What is my target INR range, and why?
How often do I need testing?
Who should I call if I am unwell or start a new medicine?
Would a DOAC be suitable for me?

Frequently asked questions

Do newer blood thinners need INR checks?

Direct oral anticoagulants (DOACs) such as apixaban do not need routine INR monitoring.

Should I stop eating greens?

No. Keep your intake consistent rather than avoiding them.

What if I miss a dose?

Follow your clinic’s advice. Often you take it as soon as you remember the same day, but never take a double dose the next day.

Can I take paracetamol with warfarin?

Paracetamol is usually preferred over anti-inflammatory painkillers, but regular high doses can raise INR. Check with your clinic.

Is it safe to get pregnant on warfarin?

Warfarin can harm an unborn baby, especially early in pregnancy. Talk to your healthcare provider before trying to conceive.

But I feel fine with a high INR

You can feel completely fine and still be at risk of serious bleeding. Follow the clinic’s advice.

Your next step: Keep a record of your INR results and tell every clinician and pharmacist that you take warfarin.

Sources: National Institute for Health and Care Excellence, British Society for Haematology, NHS, MedlinePlus.