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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 blood clots.

Quick summary

["PT/INR helps doctors monitor blood clotting disorders, especially when on anticoagulant medications."]

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

Key facts at a glance

LOINC
3289-6
Short code
Prothrom Act/Nor PPP
Synonyms
Prothrom Act/Nor PPP
Category
chemistry
Sample
plasma
Container
Lavender-top (EDTA) or Green-top (Heparin)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
coag
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 for Your Health

PT measures the time it takes for a small cut or injury to form a stable clot. The INR standardizes this measurement across labs, making it easier to compare results globally. This test is critical for people with conditions like liver disease, blood clotting disorders, or those taking blood thinners like warfarin.

How the Test Works

A small sample of your blood is mixed with a special solution that triggers the clotting process. The time taken for a clot to form is recorded. The INR is calculated using a formula that accounts for variations in normal clotting times across different labs.

Common Reasons for Testing

Doctors order PT/INR tests to check for bleeding risks, monitor anticoagulant therapy, diagnose clotting disorders, or evaluate liver function. It’s often used before surgeries or procedures that could cause bleeding.

What to Expect Before the Test

No special preparation is needed unless your doctor instructs otherwise. You may be asked to fast for 8–12 hours before the test if it involves a specific type of blood draw, but this is rare for PT/INR. Wear comfortable clothing and arrive with an ID for easy access to your medical records.

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 or low PT/INR result mean?
A prolonged PT/INR (longer clotting time) may indicate a bleeding disorder, liver disease, or insufficient blood thinning (e.g., low INR in someone on warfarin). A shortened PT/INR could suggest an increased risk of clots, though this is less common. Your doctor will interpret results in the context of your medical history and symptoms.
How often should I get this test?
If you’re on long-term anticoagulant therapy, your doctor may order PT/INR tests every few weeks or months to adjust your medication. For other conditions (e.g., liver disease), testing frequency depends on your stability and symptoms. Follow your doctor’s guidance.
Can lifestyle changes affect PT/INR results?
Yes. Certain foods (like vitamin K-rich foods like leafy greens) can influence PT/INR in people on warfarin. Smoking, alcohol, or other medications may also interact with blood thinners. Always discuss your diet, supplements, and medications with your doctor before testing.
  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