INR
International Normalised Ratio (Prothrombin Time)
What this test measures
INR measures how long your blood takes to clot. It's mainly used to monitor warfarin (a blood-thinning medication) and to check for clotting problems.
Understanding the numbers
Typical range: 0.9-1.2 (not on warfarin) | 2.0-3.0 (on warfarin, but target varies)
If you take warfarin, your target INR depends on why you're taking it. Your GP or anticoagulant clinic will tell you your personal target.
- Normal (not on warfarin): 0.8–1.2 ratio
- Target AF / VTE (on warfarin): 2.0–3.0 ratio
- Target metal valve / recurrent VTE: 2.5–3.5 ratio
- Above target — bleeding risk rising: 3.5–5.0 ratio
- High bleed risk — vitamin K often needed: 5.0–8 ratio
Key numbers
- 3.0 ratio — Upper target for AF / VTE
- 5.0 ratio — Above = hold warfarin; consider vitamin K
- 8.0 ratio — Very high — urgent hospital review
- 4.5 ratio — Check for interacting meds (antibiotics, amiodarone)
Management of high INR follows BSH and NICE guidance: INR 5-8 without bleeding — hold warfarin, restart at lower dose when INR back in range; INR >8 without bleeding — oral vitamin K; any major bleed — hospital, vitamin K + PCC. Target INR depends on indication (2.5 for AF/VTE; 3.5 for mechanical mitral valves).
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your blood is taking longer to clot (thinner blood)
- If on warfarin, your dose may need reducing
- Higher risk of bleeding (bruising, nosebleeds, blood in urine)
- Liver disease or vitamin K deficiency can raise INR
When not to wait
High INR with bleeding — or very high INR — needs urgent hospital assessment.
- Uncontrolled nose or gum bleeding
- Vomiting blood or passing black tarry stools
- Blood in urine or heavy bleeding from any cut
- Severe bruising without obvious cause
- New severe headache, weakness, or confusion
- Recent fall with a hit to the head
- Known INR above 8 without bleeding still warrants urgent medical review
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- Do I need my warfarin dose adjusted?
- What foods or medicines might be affecting my INR?
- How often should I have my INR checked?
- What signs of bleeding should I watch for?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
High INR after a new antibiotic
Edna is on warfarin for atrial fibrillation, usual target 2.5. After a course of clarithromycin, her INR came back at 6.8. No bleeding.
What the GP said: “Clarithromycin pushes warfarin up. We'll hold warfarin for two days and recheck; because your INR is between 5 and 8 without bleeding, we don't give vitamin K. When it drops below 5 we'll restart at a slightly lower dose.”
What happened next: After two days off warfarin her INR was 3.2; she restarted at 1.5 mg (previously 2 mg) and stabilised at 2.6 over two weeks.
Many medicines (some antibiotics, amiodarone, NSAIDs) raise INR on warfarin — always tell whoever is prescribing you a new medicine. Mild rises often just need a pause and recheck; significant rises follow a protocol.
Atrial fibrillation — in-target INR
Jamie has AF on warfarin. His INR on the last three visits has been 2.5, 2.8 and 2.7 — comfortably in range.
What the GP said: “You're doing well — over 70% of your readings are in range, which is what we aim for. Keep doing what you're doing and we'll check again in 8-10 weeks.”
Stable INR in range on warfarin is the goal — 'time in therapeutic range' (TTR) above 65-70% is considered good control. Predictable INR often allows longer gaps between tests.
Raised INR from liver disease
Poonam has known cirrhosis. She is not on warfarin. Her INR was 1.6, albumin 28, bilirubin 42.
What the GP said: “Your liver makes many of the clotting factors. A raised INR here reflects your liver struggling — not a medicine problem. We monitor liver function and plan carefully around any procedures.”
What happened next: Poonam continued her liver clinic reviews; her INR remained around 1.5-1.7.
INR isn't only about warfarin — it's a marker of the liver's synthetic function. Raised INR in chronic liver disease reflects worsening liver function, not a clotting problem to treat.
Critical INR with bleeding
Harpreet is on warfarin. Presented with nose bleeds and large bruises. INR was 9.5.
What the GP said: “Your INR is dangerously high and you're bleeding. This needs hospital treatment: IV vitamin K and possibly prothrombin complex concentrate. I'm sending you to A&E now.”
What happened next: Harpreet received IV vitamin K and PCC in hospital. His INR came down to 1.8 within hours and his bleeding stopped.
INR above 8, especially with any bleeding, is a medical emergency on warfarin. Reversal agents work quickly — but you must go to hospital, not stay at home.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.