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Prothrombin Time (PT) with International Normalized Ratio (INR) Auto-Detection

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

This test measures how long it takes for your blood to clot, helping assess blood clotting function and monitor certain medications.

Quick summary

["PT/INR helps track blood clotting health and medication effectiveness."]

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

Key facts at a glance

LOINC
32207-3
Short code
PDW Bld Auto
Synonyms
PDW Bld Auto
Category
hematology
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
automated-count
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

What Is PT/INR Used For?

PT measures the time it takes for blood to clot after an anticoagulant (like tissue factor) is added. INR standardizes results globally by comparing your PT to a reference sample. Together, they help diagnose bleeding disorders, evaluate liver function, and monitor treatments like warfarin.

How Is This Test Done?

A small blood sample is taken from your arm. The lab adds a specific substance to start the clotting process and measures the time it takes for a clot to form. The result is expressed as PT (in seconds) and INR (a ratio).

Why Might You Need This Test?

You may need PT/INR if you have conditions like liver disease, bleeding disorders, or if you’re taking blood thinners like warfarin. It’s also used to screen for inherited clotting disorders or monitor treatment responses.

What Does a Result Mean?

Results are interpreted by your healthcare provider based on your medical history and symptoms. Higher PT/INR times may indicate slower clotting (risk of bleeding), while lower times may suggest faster clotting (risk of clots). No specific ranges are provided here—your doctor will explain what’s normal for you.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

PT/INR Auto

Frequently asked questions

Why do I need this test if I’m not on blood thinners?
Even if you’re not on medication, PT/INR can help detect underlying bleeding risks, liver issues, or inherited clotting problems that might not cause symptoms.
How often should I get this test?
Your doctor will determine frequency based on your condition, medications, or symptoms. Regular monitoring may be needed if you have chronic liver disease, take warfarin, or have a history of clotting disorders.
Can I eat or drink before this test?
Typically, no fasting is required for PT/INR. However, follow your doctor’s instructions, as some medications or foods (like vitamin K-rich foods) may affect results.
  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