What Is the PT/INR Test?
The prothrombin time (PT) measures how quickly your blood clots. The INR (International Normalised Ratio) is a standardised calculation from the PT that allows results to be compared between labs. It is mainly used to monitor warfarin (Coumadin) therapy and assess bleeding risk.
Normal PT/INR Values
| Result | Range | Meaning |
|---|---|---|
| Normal PT | 11–13.5 seconds | Blood clots in a normal time |
| Normal INR | 0.8–1.2 | Not on anticoagulants |
| Therapeutic INR (warfarin, most conditions) | 2.0–3.0 | Target for AF, DVT/PE, mechanical mitral valve |
| Therapeutic INR (mechanical aortic valve) | 2.0–3.0 | Higher targets may apply — doctor decides |
| INR > 4.0 | Dangerously high | Serious bleeding risk |
What a High PT/INR Means
- Warfarin dose is too high — most common reason
- Liver disease (the liver makes clotting factors)
- Vitamin K deficiency
- Disseminated intravascular coagulation (DIC)
- Certain inherited clotting factor deficiencies
What a Low PT/INR Means
A low INR (below 2.0 in someone on warfarin) means the blood is clotting faster than desired — the patient may be at risk of blood clots. The warfarin dose may need adjusting.
Seek emergency care if:You are on warfarin and have an INR above 4.0 with active bleeding, or above 5.0 even without symptoms — your dose needs urgent adjustment.
Frequently Asked Questions
How often is INR tested on warfarin?
Initially weekly until stable, then monthly. More frequent if doses change.
Can diet affect my INR?
Yes — vitamin K-rich foods (leafy greens) lower INR. Keep your diet consistent.
Is fasting needed for a PT/INR test?
No fasting required.
Related Tests
Medical Disclaimer: PT/INR monitoring requires regular medical supervision. Never adjust warfarin doses without your doctor's guidance.