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Tuberculosis Skin Test Diameter Measurement (24-Hour Reading)

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

This test measures the size of a raised bump (wheal) that appears on your skin 24 hours after a small amount of a tuberculosis protein is injected. It helps determine if you’ve been exposed to tuberculosis bacteria.

Quick summary

["A positive reaction may indicate past or recent tuberculosis exposure."]

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

Key facts at a glance

LOINC
23537-4
Short code
TB Wheal 2D p Mam Tuberc Diam
Synonyms
TB Wheal 2D p Mam Tuberc Diam
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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 test involves injecting a small amount of tuberculin—a protein derived from tuberculosis bacteria—into the outer layer of your skin. Within 24 hours, you’ll have a small bump or redness where the injection was given. The size of this bump (wheal) is measured in millimeters to assess your immune response.

How the Test Works

Your healthcare provider will use a special needle to inject the tuberculin under your skin. You’ll need to return to the clinic 24 hours later for the reading. The bump’s diameter is measured to determine if an immune response (infection risk) was triggered.

Why It’s Done

This test is primarily used for screening people at higher risk of tuberculosis, such as those with weakened immune systems, recent immigrants from high-risk areas, or healthcare workers. It’s not a diagnostic test for active tuberculosis but helps identify potential exposure.

What to Expect Before the Test

No special preparation is needed. Wear loose clothing to make arm access easier. Avoid taking antihistamines or steroids 48 hours before the test, as they may affect results.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

TST Diameter (24h)

Frequently asked questions

Why did I need this test?
Your doctor may have recommended this test if you have risk factors for tuberculosis, such as living in a high-risk area, having a weakened immune system, or being in close contact with someone who has tuberculosis.
What does a positive or negative result mean?
A positive result (visible bump) means your body may have been exposed to tuberculosis bacteria, but it doesn’t confirm active disease. A negative result means no visible reaction, so no immediate risk is detected. Follow-up testing may be needed if your doctor suspects exposure.
Can I take anything before or after the test?
Yes, avoid antihistamines or steroids 48 hours before the test. Afterward, you can return to normal activities, but avoid picking at the injection site to prevent infection.
  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