Hematologist
CION Cancer Clinics
Warfarin and the INR test, in plain language | CION Cancer Clinics
The INR is a blood test that shows how long your blood takes to clot on warfarin. For most people treated for a clot in the leg or lungs, the target is between 2 and 3. Too low risks a new clot; too high raises bleeding risk. This page explains how testing works, what moves the number, and what your team decides. At CION Cancer Clinics, our haematologist cares for anaemia, bleeding, clotting and inherited blood disorders, with ArogyaSri, CGHS and cashless insurance accepted.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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On this page
- What is an INR test, and why does warfarin need it?
- How does INR monitoring actually work?
- What can push your INR up or down?
- What do the words on an INR report mean?
- What do people often get wrong about warfarin?
- Who does warfarin not suit, and what can this page not tell you?
- Common questions about warfarin and INR tests
The short answer
What is an INR test, and why does warfarin need it?
The INR is a blood test that shows how long your blood takes to clot while you are on warfarin. For most people treated for a clot in the leg or lungs, the team aims to keep the INR between 2 and 3.
Warfarin is a blood thinner that has been used for decades. It works well, but the right amount differs a great deal from person to person, and it can change from week to week in the same person. The INR is how your team finds and keeps the right amount for you.
What the number means
A person not on warfarin usually has an INR of about 1. A higher number means the blood takes longer to clot. Too low, and a new clot can form. Too high, and the risk of bleeding goes up. The target range sits between the two.
Why your target may be different
Some people are given a higher target, for example after certain artificial heart valve operations or a repeat clot while already on treatment. Your target is written on your warfarin record book or slip. If it is not, ask.
Reference ranges and targets can differ between laboratories and doctors. A single INR is always read alongside your symptoms, your recent results and any change in your medicines.
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When to go to hospital the same day
If you are on warfarin and have bleeding that will not stop, vomit blood, pass black or red stools, have red or dark brown urine, or get a severe sudden headache, go to the nearest emergency department now or call 108. Go too after a fall or a blow to the head, even if you feel fine. Take your warfarin book and your latest INR result with you. Do not skip or change your warfarin on your own to try to fix it.
Not sure whether this applies to you?
Ask an oncologistThe routine
How does INR monitoring actually work?
Frequent checks at the start
When warfarin is started, the INR is checked often, sometimes every few days. The amount is adjusted by your team each time until the number settles inside your target range.
The gap widens
Once your INR has stayed steady for a while, the tests are spaced further apart. People who stay stable for a long time may only need a test every few weeks.
The test itself
It is a small blood sample, either from a vein or from a finger prick at a clinic that has the machine. You do not need to fast. Tell the person taking it if you have missed a dose.
The plan comes back to you
Your team tells you what to take until the next test and writes it in your book. Keep the book with you and bring it to every visit, including to other doctors and your dentist.
Why the number moves
What can push your INR up or down?
Most unexpected results have an ordinary cause. Tell the team about any of these before your next test, not after.
Food, especially greens
Vitamin K helps blood clot, and it works against warfarin. Leafy greens such as palak, methi and cabbage are rich in it. You do not have to avoid them. The aim is to eat a similar amount each week, without sudden changes.
Other medicines
Many common medicines change the INR. Always tell the prescriber you take warfarin before starting anything new.
Often involved
- Antibiotics and antifungal medicines
- Painkillers such as aspirin or ibuprofen
- Ayurvedic, herbal and homeopathic products
Illness
A fever, loose motions, vomiting or not eating for a few days can all make the INR swing, usually upwards. Call the team if you are unwell rather than waiting for your routine test.
Alcohol and missed doses
Heavy or binge drinking can push the INR up sharply. Missed or doubled doses make the next result hard to read. Note any missed dose in your book and tell the team.
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On your report
What do the words on an INR report mean?
- PT (prothrombin time)
- The time, in seconds, your blood sample takes to clot in the laboratory. The INR is calculated from it.
- INR
- The PT turned into a standard number, so a result from one laboratory can be compared with another.
- Target range
- The band your doctor wants your INR to stay within. It is set for you, not printed as a general normal range.
- Sub-therapeutic
- Below your target. The blood is clotting more easily than your team wants.
- Supra-therapeutic or high
- Above your target. The chance of bleeding is higher, and the team will usually contact you about it.
Commonly believed
What do people often get wrong about warfarin?
"I must stop eating green vegetables."
Cutting them out can make the INR swing when you eat them again. Keep your usual diet steady. Your team sets the warfarin around what you normally eat.
"My INR was high, so I skipped a few tablets."
Changing the amount on your own is one of the commonest reasons the INR swings the other way. A high result should be reported to your team, who decide what to do. If you are bleeding, go to hospital.
"Warfarin is rat poison, so it must be dangerous."
The same family of chemicals is used in rat poison, but the amount given as a medicine is small and carefully watched. The INR test is exactly what makes it safe to use.
"Once my INR is steady, I no longer need tests."
A new medicine, an illness or a change in diet can move a steady INR at any time. The gap between tests grows, but the tests continue for as long as you take warfarin.
Being straight with you
Who does warfarin not suit, and what can this page not tell you?
Warfarin is not the right choice for everyone. It is usually avoided in early pregnancy, because it can harm the baby. It is harder to use safely for people who cannot reach a clinic for regular tests, who have had serious bleeding before, or whose diet and drinking vary a lot.
The newer blood thinners
Many people with a clot are now offered one of the newer tablets, often called DOACs, which do not need INR tests. They do not suit everyone either. They are generally not used with certain artificial heart valves, and some are not suitable with poor kidney function. Your doctor weighs these against cost and your other medicines.
What this page cannot tell you
It cannot tell you your own target or what to take. A number on one slip does not tell the whole story. Your team reads it against your earlier results and anything that has changed. If you are caring for a parent on warfarin, keep a list of every medicine and bring it to each visit.
CION's haematology team can review your INR record and your medicines, and tell you what to ask your treating doctor.Questions we are asked
Common questions about warfarin and INR tests
My INR is slightly high. Should I worry?
A result a little above your range, with no bleeding, is common and is usually handled by your team adjusting the plan. Call them the same day and follow their advice. Do not change your tablets on your own. If you have any bleeding that will not stop, or a severe headache, go to hospital straight away.
My INR is low. Am I at risk of a clot?
A low result means you are less protected than your team wants. It is not an emergency on its own, but tell them promptly so they can decide what to do. Think back over the past week for missed doses, new medicines or a change in diet, and mention it. Seek care at once for a swollen painful leg or sudden breathlessness.
What time of day should I take warfarin?
Most people are asked to take it at the same time every day, often in the evening. The evening timing lets the team change the plan the same day after a morning blood test. Follow the timing your team gives you, and keep to it, because regular timing helps keep the INR steady.
What should I do if I miss a dose?
Do not take a double amount to make up for it. Write the missed dose in your warfarin book and call your team for advice on what to do next. Tell the person taking your next INR test, because a missed dose can make the result look lower than your usual level.
Can I eat my normal Indian diet on warfarin?
Yes. The aim is steadiness, not avoidance. Eat palak, methi and other greens in roughly the same amounts each week. Take care with sudden changes such as a fast, a festival week, a new diet or large amounts of green tea. Tell your team before you plan a big change in what you eat.
Can I take a painkiller for a headache or body ache?
Ask first. Some common painkillers, including aspirin and ibuprofen, raise the bleeding risk on warfarin. Paracetamol is often considered the safer option, but regular use can still affect the INR. Your doctor or pharmacist can tell you what is suitable for you.
Do I need to tell my dentist I am on warfarin?
Yes, every time. Many dental treatments can go ahead with warfarin continued, but the dentist may want a recent INR result first. Extractions and other surgery may need planning with your doctor. Never stop warfarin yourself before a procedure. Your doctors agree that plan together.
Could I switch to a tablet that needs no blood tests?
Possibly. Many people with a clot can use one of the newer blood thinners instead. They do not suit everyone, including some people with artificial heart valves, poor kidney function or certain other conditions. Ask your haematologist at your next review whether a switch would be safe for you.
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Sources
- NHS — Warfarin
- NICE — Anticoagulation - oral (Clinical Knowledge Summaries)
- American Society of Hematology — Blood Clots
- NHLBI — Venous Thromboembolism
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
DOACs vs WarfarinHow the newer tablets compare, and who they do not suitMissed Anticoagulant Dose: What to DoWhat to tell your team after a missed tabletBleeding Risk on AnticoagulantsThe bleeding signs to watch for on warfarinHow Long to Stay on Blood ThinnersHow long a course usually lasts after a clot
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Struggling to keep your INR steady?
Share your INR record and medicine list with us. CION's haematology team will go through it and tell you what to ask your treating doctor. One helpline serves every CION centre.