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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 potential bleeding or clotting disorders.

Quick summary

["PT/INR helps monitor treatments for blood clotting disorders and ensures safe medication dosing."]

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

Key facts at a glance

LOINC
18520-7
Short code
P-R interval L-AVF
Synonyms
P-R interval L-AVF
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

What Is PT/INR Used For?

PT/INR is primarily used to evaluate the effectiveness of blood-thinning medications like warfarin. It also helps diagnose bleeding or clotting problems, such as those caused by liver disease or certain blood disorders.

How Is the Test Performed?

A small sample of your blood is drawn and mixed with a specific chemical to measure how long it takes to clot. The INR is then calculated to compare your results to a standard reference.

Why Might You Need This Test?

You may need a PT/INR test if you’re taking blood-thinning medications, have a history of clotting or bleeding disorders, or if you have liver disease. It’s also used before surgeries to ensure safe blood clotting levels.

What Can Affect Your Results?

Factors like certain medications (both blood thinners and others), liver disease, infections, or even stress can influence PT/INR results. Always inform your healthcare provider about all medications or health conditions.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

PT/INR

Frequently asked questions

How long does it take to get results from this test?
Results are typically available within 1 to 3 business days, depending on your lab’s processing time. Your doctor will discuss the findings with you during your follow-up visit.
What do abnormal PT/INR results mean?
Elevated PT/INR may indicate bleeding risks or insufficient blood clotting, while prolonged times could signal liver issues or medication overdosing. Your doctor will interpret results in the context of your health and treatment plan.
Can lifestyle changes affect PT/INR?
Yes, certain lifestyle factors like diet (especially vitamin K-rich foods), alcohol consumption, or physical activity can influence blood clotting. Always follow your doctor’s guidance on managing these factors.
  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