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International Normalized Ratio (INR) with Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT) Monitoring

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

This test measures how long it takes for your blood to clot, helping doctors monitor blood-thinning medications like warfarin.

Quick summary

["Your INR helps track how well your blood is controlled to prevent clots or bleeding risks."]

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

Key facts at a glance

LOINC
92891-1
Short code
Coagulation.INR goal PPP PT
Synonyms
Coagulation.INR goal PPP PT
Category
coagulation
Sample
plasma
Container
Lavender-top (EDTA) or Green-top (Heparin)
Units
N/A
Ref low
44
Ref high
147
Critical range
— – 5 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
coag-tissue-factor-induced
CPT
84075
SNOMED
27550007
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 This Test Measures

The INR combines results from two key clotting tests: Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). These tests check different parts of your blood’s clotting process. Your INR is calculated to compare your results to a standard, ensuring consistent monitoring across labs.

Why It’s Done

Doctors order this test if you’re on blood-thinning medications (like warfarin), have certain blood disorders, or are at risk of bleeding or clotting. It helps adjust medications to keep your blood within a safe balance.

How It Works

A small blood sample is taken from your arm. The lab measures how long it takes for your blood to clot under controlled conditions. The INR is then calculated based on your PT and aPTT results compared to a standard.

What to Expect Before the Test

No special preparation is needed, but inform your doctor about any medications (especially blood thinners), recent injuries, or illnesses that might affect your blood.

Reference range

Adult: 44-147 U/LN/A

Specimen: plasma

No special preparation required. Avoid fatty meals before the test if possible.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

INR Monitoring

Frequently asked questions

What does an abnormal INR result mean?
An abnormal INR may indicate your blood is clotting too slowly (high INR) or too quickly (low INR). This could signal issues like vitamin K deficiency, liver disease, or improper medication dosing. Your doctor will discuss findings with you.
Can I eat or drink before this test?
You can eat and drink normally unless instructed otherwise. Avoid alcohol before testing, as it may affect blood clotting.
How often will I need this test?
Frequency depends on your treatment. For example, patients on warfarin may need INR checks weekly, monthly, or as needed to adjust doses.
  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