INR (prothrombin time)
Also called: INR, International normalised ratio, International normalized ratio, Prothrombin time, PT/INR, PT
A measure of how long blood takes to clot along one of its two pathways. Its main jobs are to monitor the blood-thinner warfarin and to assess the liver's clotting-factor production, since most of those factors are made in the liver.
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What does INR (prothrombin time) measure?
The INR (international normalised ratio) is a standardised version of the prothrombin time — the seconds it takes plasma to clot through the 'extrinsic' pathway. Standardising it as a ratio lets results be compared between laboratories. Its two big uses are: monitoring warfarin, a blood thinner whose dose is titrated to keep the INR in a target range (usually 2.0–3.0); and gauging liver function, because the liver makes most clotting factors and several depend on vitamin K, so a rising INR is a sensitive sign of failing liver synthesis or vitamin K deficiency. A high INR means blood is slower to clot (more 'thinned'); a low INR in someone on warfarin means under-anticoagulation and clot risk. It is read alongside APTT (the other clotting pathway) and, in liver disease, albumin and bilirubin.
What does a high INR (prothrombin time) level mean?
- Warfarin effect — the intended action; too high means over-anticoagulation and bleeding risk.
- Liver disease — reduced synthesis of clotting factors; a rising INR tracks worsening liver function.
- Vitamin K deficiency — poor diet, malabsorption, some antibiotics.
- Disseminated intravascular coagulation (DIC) — clotting factors consumed in serious illness.
- Direct oral anticoagulants and some other drugs — can prolong it (though these aren't monitored by INR).
What does a low INR (prothrombin time) level mean?
- Normal off treatment — an INR near 1.0 is expected without anticoagulation.
- Under-anticoagulation on warfarin — below the target range means insufficient thinning and a clot risk.
- Rarely — an increased clotting tendency; not usually diagnosed from the INR alone.
When to discuss with a doctor
The INR is only meaningful once you know why it was done. On warfarin, it guides dosing: too high raises bleeding risk (and a very high INR with any bleeding needs urgent care and reversal), too low leaves you under-protected from clots — both are reasons to contact the clinic managing your warfarin. Off anticoagulation, an unexpectedly high INR is a meaningful sign of liver dysfunction or vitamin K deficiency and should be evaluated. Mediora.AI reads the INR against its target where warfarin is involved, and alongside the liver markers otherwise; dose changes and the clinical decision belong with the team managing your anticoagulation or liver care.
Related markers
Commonly confused with
- Prothrombin time (PT) The INR is not a separate test: it is the prothrombin time converted so that results from different laboratories can be compared. That is why a dose of an anticoagulant is set by the INR and not by the seconds.