This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
What the INR measures
The prothrombin time (PT) measures how long blood plasma takes to clot after a reagent called thromboplastin is added. Because thromboplastins differ in sensitivity, the same blood sample could give different PT results in different laboratories. In 1983 the World Health Organization introduced the international normalized ratio (INR), which corrects for this:
INR = (patient PT ÷ mean normal PT) ^ ISI
- The mean normal PT is the laboratory’s average PT in healthy people, typically around 11 to 13 seconds.
- The ISI (International Sensitivity Index) describes the reagent’s sensitivity compared with a WHO reference. Modern reagents have an ISI close to 1.0.
Target ranges on warfarin
| Situation | Usual INR target |
|---|---|
| No anticoagulant | about 0.8 to 1.2 |
| Atrial fibrillation, deep vein thrombosis, pulmonary embolism, bioprosthetic valve | 2.0 to 3.0 |
| Mechanical mitral valve, or mechanical aortic valve with extra risk factors | 2.5 to 3.5 |
Below the target, protection against clots is reduced; above it, the risk of bleeding rises, especially above 4.5.
Example
A patient with a PT of 30 seconds, a mean normal PT of 12 seconds and a reagent ISI of 1.0 has an INR of (30 ÷ 12)^1.0 = 2.5, in the middle of the usual target range.
What changes the INR
- Diet: sudden changes in foods rich in vitamin K, such as kale, spinach and broccoli. Keep your intake steady rather than avoiding them.
- Drugs: many antibiotics, antifungals, amiodarone and some supplements raise the INR; others lower it. Tell every prescriber and pharmacist that you take warfarin.
- Illness: fever, diarrhea, liver disease and heart failure can raise it.
- Alcohol: binge drinking can raise the INR.
Monitoring
INR tests are usually done every few days when warfarin is started, then every 4 to 12 weeks when stable. Some patients test at home with a fingerstick meter. Direct oral anticoagulants such as apixaban or rivaroxaban do not need INR monitoring.
Seek urgent care for unusual bleeding, black stools, blood in urine, a severe headache or a fall with a head injury while on warfarin.