Short answer: The prothrombin time (PT) measures how many seconds a blood sample takes to clot after a reagent is added. Because reagents differ between labs, the result is converted to the INR (international normalised ratio), which can be compared anywhere. INR is checked regularly in people taking vitamin K antagonist medicines such as warfarin, and also to assess the liver's production of clotting factors.
At a glance
| PT | Seconds a sample takes to clot after a reagent is added |
|---|---|
| INR | PT converted to a ratio that can be compared between labs |
| Ordered for | People on vitamin K antagonists such as warfarin; liver assessment; before surgery |
| LOINC | 5902-2 (PT), 6301-6 (INR) |
| Moves with | Diet (vitamin K), other medicines, alcohol, illness |
| Keep | INR with the date and the dose taken; the doctor sets the target range |
What the test measures
PT reflects several clotting factors made by the liver, some of which need vitamin K. The INR is the patient's PT divided by a normal reference PT, adjusted for the sensitivity of the reagent; a person not taking anticoagulants usually has an INR around 1.
For people on vitamin K antagonists, the doctor sets a target range (often between 2 and 3, sometimes higher), and doses are adjusted to stay in it. Newer anticoagulants (DOACs) are not monitored with INR.
When it is ordered
Common reasons, which your doctor adapts to you:
- regularly, for people taking warfarin or similar medicines;
- before surgery or invasive procedures;
- to assess bleeding problems or liver function.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| INR | 6301-6 | ratio (no unit) | — |
| Prothrombin time (PT) | 5902-2 | seconds | % of normal (some countries) |
INR has no unit: it is a ratio. PT is in seconds; some European labs also report it as a percentage of normal activity (Quick value), which moves in the opposite direction to the INR.
Reading your report
Your target range is personal and set by your clinician; the range printed on a general report often applies to people not taking anticoagulants. Diet (vitamin K in green vegetables), alcohol, illness and many medicines and supplements change the INR, which is why people on warfarin keep a log of results, doses and changes. Never change the dose yourself based on a result.
Keeping the results in your record
Scan or photograph the report in Anpheros Daily: each value is read with its unit and the laboratory's reference range, you check it, and it is stored with its LOINC code as an HL7 FHIR Observation, a format used by many hospitals and national health systems. Results from different laboratories line up on one chart over time, and the lab unit converter helps when two reports use different units. Our guide on how to read lab results explains reference ranges and flags in general; the lab results guide for a new doctor shows how to share them.
What this page does not do
It explains what the test is and how the result is written. It does not interpret your result, diagnose or recommend treatment: the meaning of a value depends on your history, your other results and how the sample was taken, and that is your doctor's job.
Related pages
- Liver function tests
- Complete blood count (CBC)
- One medication list for the whole family, ready for any doctor or pharmacy
Sources
Frequently asked questions
What is the difference between PT and INR?
PT is the clotting time in seconds; INR standardises it so results from different labs are comparable.
What INR should I have?
Only your doctor can say. People not on anticoagulants are usually around 1; people on warfarin have a personal target range, often 2–3.
Do I need INR checks on newer anticoagulants?
Usually not. DOACs such as apixaban or rivaroxaban are not monitored with INR. Ask your doctor.
Can food change my INR?
Yes. Vitamin K in leafy greens, alcohol and many medicines and supplements can change it. Ask your clinic how to handle your diet, and tell them about changes in food, alcohol, medicines or supplements.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
How it works in the app →
Anpheros builds a medical record that the patient controls and can share with any clinic, on an HL7 FHIR R4 platform, based in the EU. These guides help you understand and organise your health; they do not replace your doctor.
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