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

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 and treatment effectiveness."]

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

Key facts at a glance

LOINC
77161-8
Short code
Prothrom imm FII DP PPP-aCnc
Synonyms
Prothrom imm FII DP PPP-aCnc
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

What Does This Test Measure?

The Prothrombin Time (PT) test checks the time it takes for blood to clot under controlled conditions. The International Normalized Ratio (INR) standardizes these results to compare across labs. Together, they evaluate the function of clotting factors in your blood, particularly those involved in the extrinsic pathway.

Why Is This Test Done?

Your doctor may order a PT/INR test if you have conditions like liver disease, blood clotting disorders (e.g., hemophilia), or are taking blood thinners (e.g., warfarin). It’s also used to monitor treatment progress in patients with clotting risks.

Sample Collection and Preparation

This test uses a small sample of your blood drawn from a vein. The blood is mixed with a special reagent to simulate clotting conditions. The time taken for clotting is measured and converted to an INR value for comparison to standard ranges.

Common Reasons for Abnormal Results

Elevated PT/INR may indicate liver disease, vitamin K deficiency, or blood thinners working too strongly. Low PT/INR could suggest rare clotting disorders or overuse of anticoagulants. Always discuss results with your healthcare provider.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

PT/INR

Frequently asked questions

How often will I need this test?
If you’re on blood thinners, your doctor may schedule PT/INR tests regularly (e.g., weekly or monthly) to adjust your medication. For other conditions, tests may be done less frequently, depending on your health status.
What do I do if my PT/INR is out of range?
If your results are abnormal, your doctor will review your medical history and other tests to determine the cause. They may adjust medications, recommend dietary changes (e.g., vitamin K), or suggest further testing.
Can I eat or drink before this test?
For PT/INR tests, avoid large amounts of vitamin K-rich foods (e.g., leafy greens) 2–3 days before testing if on blood thinners. Follow your doctor’s specific instructions for fasting or diet restrictions.
  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