Prothrombin time and INR
Also known as: clotting time test, warfarin blood test, international normalized ratio
The prothrombin time measures how many seconds plasma takes to clot once the tissue-factor pathway is triggered; the INR expresses that time as a ratio standardised across laboratories. It is the test that tunes the dose of warfarin and one of the quickest measures of how well the liver is making clotting factors.
- Canonical name
- Prothrombin time and international normalised ratio
- Abbreviations
- PT, INR, PT/INR
- Kind
- Laboratory test
- Categories
- Coagulation & haemostasis, Liver & hepatobiliary, Point-of-care & home testing
- Specimen
- Plasma, Capillary (fingerprick) blood
- Method
- Clot-based coagulation assay, Point-of-care device
- Terminology
- LOINC 5902-2, 6301-6
- Type
- Blood test (coagulation)
- Sample
- Blood from a vein in a citrate tube; fingerprick meter for home INR
- Used for
- Dosing warfarin, assessing liver synthetic function, investigating bleeding
- Measures
- How long plasma takes to clot through the tissue-factor pathway, standardised as the INR
- Result
- Numeric: seconds (PT) and a ratio (INR), read against the person's target
Why is it used?
- Monitoring warfarin and deciding its dose
- Assessing the liver's synthetic function in liver disease
- Unexplained bleeding or bruising, and before surgery or a biopsy
- Suspected vitamin K deficiency or disseminated intravascular coagulation
What does it measure?
| Component | What it measures | A higher value may be seen in | A lower value may be seen in |
|---|---|---|---|
| Prothrombin time PT | Seconds to clot through the tissue-factor pathway | Warfarin, liver disease, vitamin K deficiency, DIC | Not usually clinically significant |
| INR INR | The prothrombin time standardised across laboratories | Warfarin effect above target, liver failure, vitamin K deficiency | Warfarin effect below target |
A high or low value can be associated with several different situations; the result is interpreted with other results, symptoms, history and clinical findings.
How is it performed?
A venous blood sample taken into a citrate (blue-top) tube, filled to the line so the anticoagulant-to-blood ratio is right; people on warfarin can use a fingerprick meter at home or in an anticoagulant clinic.
Specimen. Venous blood in a citrate tube, filled correctly; capillary blood for point-of-care INR meters
Do I need to prepare?
None. Tell the laboratory or clinic about all medicines, supplements and any recent change in diet, illness or alcohol intake, which change the result.
What do the results generally mean?
| Finding | What it may indicate |
|---|---|
| INR within the target range set for the person on warfarin | The dose is giving the intended degree of anticoagulation; the target depends on why warfarin is taken |
| INR above the target range | Anticoagulation is stronger than intended and bleeding risk rises; the dose is reviewed, and a very high INR or bleeding is treated urgently |
| INR below the target range | Anticoagulation is weaker than intended; a missed dose, a new medicine or a change in diet is looked for |
| Prolonged PT in someone not taking warfarin | May reflect liver disease, vitamin K deficiency, a clotting-factor deficiency or disseminated intravascular coagulation, interpreted with the aPTT and other tests |
Guideline thresholds
| Threshold | Value | Unit | Applies to | Country · source |
|---|---|---|---|---|
| Usual INR target on warfarin | 2.0 to 3.0 | INR | The target range for most people taking warfarin, for example for atrial fibrillation or a venous clot; some mechanical heart valves need a higher range, set individually | UK UK |
Clinical decision thresholds come from the named guideline and country; they are not laboratory reference intervals, and they change when guidance changes.
What can affect the result?
- Warfarin dose, missed doses and how consistently it is taken
- Vitamin K intake (green leafy vegetables) and supplements
- Antibiotics, antifungals, amiodarone, some antiepileptics and many other interacting medicines
- Acute illness, diarrhoea, alcohol binges and liver disease
- An under-filled citrate tube, which prolongs the result artificially
Medicines. Warfarin is the medicine the test monitors; many antibiotics, antifungals, amiodarone, NSAIDs and alcohol raise the INR, while rifampicin, carbamazepine and large amounts of vitamin K lower it.
What can the test not tell you?
- How well a direct oral anticoagulant (apixaban, rivaroxaban, dabigatran) is working: the INR is not calibrated for them
- Whether bleeding will happen at a given INR, which also depends on age, other medicines and the reason for anticoagulation
- Platelet problems, which need a platelet count and function tests
The INR is only meaningful for vitamin K antagonists such as warfarin; direct oral anticoagulants prolong it unpredictably and need different tests. The prothrombin time depends on the reagent and is standardised only through the INR calculation.
Other tests commonly used with it
Catalogued concepts this page covers
Concepts of the master test taxonomy that resolve to this page; each links to its category.