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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 blood to clot, helping assess blood clotting function and identify issues with certain blood proteins.

Quick summary

["PT/INR checks for problems with blood clotting that may require treatment."]

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

Key facts at a glance

LOINC
18526-4
Short code
P-R interval L-V1
Synonyms
P-R interval L-V1
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ekg
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 Is Done

Your doctor may order a PT/INR test to evaluate blood clotting ability, especially if you have conditions like liver disease, blood clotting disorders, or are taking blood thinners like warfarin. It can also help monitor treatment effectiveness or detect bleeding risks.

How the Test Works

A small blood sample is taken and mixed with a special reagent. The time it takes for the blood to form a clot is measured, then compared to a standard to calculate the INR value.

What to Expect Before the Test

No special preparation is needed, but inform your doctor about medications (especially blood thinners) or recent injuries, as they may affect results.

What to Expect During the Test

A healthcare provider will insert a needle into your arm to draw blood. You may feel a brief pinch, but the process usually takes just a few minutes.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

PT/INR

Frequently asked questions

What does a high PT/INR mean?
A high PT/INR suggests impaired blood clotting, which could be due to liver problems, vitamin K deficiency, or certain medications. Your doctor will discuss next steps based on your overall health and symptoms.
How often should I get this test?
Frequency depends on your condition and treatment. If you’re on blood thinners, your doctor may order this test weekly, monthly, or as needed to adjust medication.
Can diet affect PT/INR results?
Yes, certain foods high in vitamin K (leafy greens, broccoli) can influence results. Your doctor may recommend limiting these temporarily if prescribed blood thinners.
  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