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INR
A standardized measure of blood clotting based on prothrombin time. The key test for monitoring warfarin and other vitamin K antagonists.
Other names: INR, International Normalized RatioSection: Blood clotting
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortThe INR is a standardized clotting measure derived from prothrombin time. A typical range for people not on blood thinners is 0.8–1.1; on warfarin, doctors usually target 2.0–3.0. Higher than target raises bleeding risk, lower raises clot risk.
Reference ranges
Group
Reference range
People not on blood thinners
0.8–1.1
Typical target range on warfarin
2.0–3.0
The source gives: INR 0.8–1.1 without blood thinners (prothrombin time 11–13.5 seconds); on warfarin the doctor usually keeps INR at 2.0–3.0, with higher targets for some conditions such as mechanical heart valves. Only your doctor sets your target range. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
The INR (international normalized ratio) is the prothrombin time recalculated with a universal formula that accounts for each laboratory's reagent. Standardization makes results comparable between labs, unlike the raw prothrombin time in seconds.
The test reflects the extrinsic clotting pathway: factors II, V, VII and X plus fibrinogen, all made in the liver with vitamin K. The higher the INR, the longer blood takes to clot. Its main use is warfarin management: the dose is adjusted to keep the INR inside the target range set for your specific condition.
When it is high
Warfarin therapyThe intended effect of the drug. Above-target values raise bleeding risk and call for a dose adjustment.
Vitamin K deficiencyVitamin K is needed to build clotting factors. Poor intake, malabsorption or antibiotics prolong clotting time.
Liver diseaseClotting factors are made in the liver — hepatitis and cirrhosis reduce production and raise the INR.
Drug and food interactionsMany medicines (antibiotics, antiarrhythmics) and sudden changes in leafy green intake strengthen or weaken warfarin.
When it is low
Missed warfarin dosesA below-target INR means insufficient protection against clots — the dose needs review.
Excess dietary vitamin KA sudden increase in leafy greens blunts warfarin and lowers the INR.
HypercoagulabilityIn thrombophilias and some inflammatory states blood clots faster than usual.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Warfarin and other anticoagulantsDietary vitamin KAntibiotics and other medicinesLiver disease
How to prepare
Blood is drawn into a citrate tube. No special diet is needed, but test at roughly the same time of day and keep your diet (especially greens and vitamin K foods) stable while on warfarin. Tell the lab and your doctor about every medicine you take.
When a retest makes sense
On warfarin — on your doctor's schedule, usually every 2–4 weeks once stable; sooner after medication changes, bleeding or unusual bruising.
Who usually reviews this result
A result outside the reference range is usually reviewed by: cardiologist, GP or phlebologist (on warfarin — the prescribing doctor); a hematologist if abnormal without anticoagulants. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: INRInternational Normalized RatioProthrombin ratio
Units
Measured in: INR (dimensionless ratio)
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
Prothrombin time is the seconds to clot and depends on the lab's reagent. The INR is the same measurement recalculated by an international formula, so it can be compared between labs and tracked over time.
How often should INR be checked on warfarin?
Every few days while the dose is being set, then usually every 2–4 weeks once stable. Any bleeding, unexplained bruising or a medication change warrants an unscheduled test.
Can I eat greens on warfarin?
Yes — consistency matters more than restriction. Sudden changes in vitamin K foods (spinach, broccoli, green tea) shift the INR, so keep intake steady.
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