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Prothrombin Time (PT) with International Normalized Ratio (INR) – Plasma

Published: August 9, 2026Updated: August 16, 2026

This test measures how long it takes for your blood to clot, helping assess your risk of bleeding or clotting disorders. It’s often used to monitor medications like warfarin.

Quick summary

["PT/INR checks blood clotting function and guides treatment for blood thinners."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
23770-1
Short code
PPV Ab Titr Ser
Synonyms
PPV Ab Titr Ser
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Matters

PT/INR is a key test for evaluating blood clotting ability. It uses a standardized method to compare your blood’s clotting time to a reference sample, expressed as an INR value. This helps doctors determine whether your blood is too thin (increased risk of bleeding) or too thick (increased risk of clots).

How It’s Done

A small blood sample is taken from a vein, usually in your arm. The blood is mixed with a special reagent to measure clotting time. The INR result is calculated based on your clotting time compared to a standard reference sample.

What It Measures

PT measures the time it takes for fibrin (the protein that forms clots) to develop in your blood. The INR adjusts this time to account for differences in testing equipment and reagents, providing a consistent measure across labs.

Common Uses

PT/INR is used to monitor people on blood thinners (e.g., warfarin) to prevent clots, diagnose bleeding disorders, or check liver function (since the liver makes clotting factors).

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

PT/INR

Frequently asked questions

What does a high INR mean?
A high INR suggests your blood clots more slowly than normal, which may indicate bleeding risks, liver disease, or vitamin K deficiency. Your doctor will adjust treatment if needed.
What does a low INR mean?
A low INR means your blood clots faster than normal, potentially raising clot risks. This could happen with certain medications, clotting disorders, or liver issues.
Why might my INR fluctuate?
Fluctuations can occur due to changes in medications, diet (e.g., vitamin K from leafy greens), liver health, or other health conditions. Your doctor will review your full medical picture.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy