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Serum or plasma concentration of **prothrombin (factor II) activity** (measured as International Normalized Ratio, INR)

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

This test measures how long it takes for blood to clot, helping assess blood clotting function and risks of bleeding or clotting disorders.

Quick summary

["INR helps monitor medications that affect blood clotting, such as warfarin."]

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

Key facts at a glance

LOINC
9936-6
Short code
Q wave Dpth L-V6
Synonyms
Q wave Dpth L-V6
Category
coagulation
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0.8
Ref high
1.2
Critical range
— – 5 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ekg
CPT
85610
SNOMED
396252006
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 an INR Test Used For?

An INR test is primarily used to evaluate the effectiveness of anticoagulant medications (like warfarin) in preventing blood clots. It also helps diagnose bleeding disorders or clotting issues in conditions such as liver disease or certain heart conditions. Your doctor may order this test if you have a history of clotting problems or if you’re taking medications that thin your blood.

How Is the INR Test Performed?

A small blood sample is drawn from a vein, usually in your arm. The blood is sent to a lab, where it’s mixed with a specific chemical to measure how long it takes to clot. The result is compared to a standard to calculate your INR value.

What Does an INR Result Mean?

Your INR result tells your doctor how your blood clotting is working compared to a healthy person. Higher INR values may indicate increased bleeding risk, while lower values could suggest a higher risk of clotting. Your doctor will interpret the result in the context of your medical history and treatment plan.

Common Reasons for an INR Test

Common reasons for an INR test include monitoring long-term anticoagulant therapy (like warfarin), diagnosing bleeding disorders, or assessing liver function. It may also be used before surgeries or procedures that carry a risk of bleeding.

Reference range

INR: 0.8-1.2 (warfarin: 2.0-3.0)N/A

Specimen: serum

Inform your healthcare provider if you are taking blood thinners. No special preparation required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

INR (Prothrombin Time)

Frequently asked questions

Why might my doctor order an INR test?
Your doctor may order an INR test if you’re taking blood-thinning medications (like warfarin), have a history of clotting or bleeding problems, or are undergoing treatment for conditions such as atrial fibrillation or heart valve disease. It’s also sometimes checked before certain surgeries or procedures.
What should I expect before or after the test?
You can eat and drink normally before the test, but avoid taking blood-thinning medications (if prescribed) unless instructed otherwise. After the test, you may experience mild bruising or soreness at the needle site. Rest and apply pressure to the area to help reduce any discomfort.
Can lifestyle factors affect my INR results?
Yes, certain foods, medications, and health conditions can influence your INR results. For example, high vitamin K intake (found in leafy greens) or certain supplements may affect clotting. Always inform your doctor about all medications, including over-the-counter drugs and herbal supplements, to ensure accurate monitoring.
  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