What Does a Positive TB Test Look Like? Bump Size Guide

If you are wondering what does a TB positive test look like, the most important thing to know is that a positive TB skin test is identified by a firm, raised or hardened area called induration at the injection site.

The test is not considered positive because the skin looks red. A healthcare professional measures the hardened area in millimeters, usually 48 to 72 hours after the test is placed.

Depending on your TB risk factors, an induration of 5 mm, 10 mm or 15 mm or more may be considered positive.

A positive skin test suggests that your immune system has reacted to tuberculosis-related antigens, but it does not automatically mean you have active tuberculosis disease.

This guide explains what a positive TB test may look and feel like, how the bump is measured, what different sizes mean, and what usually happens after a positive result.

What Does a Positive TB Test Look Like?

A positive TB skin test usually has a firm, raised, hardened area at the site where the test material was injected.

Common features may include:

A raised or slightly elevated area
Firmness under the skin
A clearly defined hardened bump
Swelling that can be felt when touched
Redness around the area in some people

The most important feature is induration, not redness.

A healthcare professional feels the skin and measures the width of the hardened area across the forearm.

Positive TB Test Quick Answer

A positive TB skin test typically shows measurable induration at the injection site.

The size considered positive depends on your risk level:

Induration Size	When It May Be Considered Positive
5 mm or more	Certain high-risk individuals
10 mm or more	People with specific TB risk factors
15 mm or more	People with no known TB risk factors

The result must be interpreted by a qualified healthcare professional.

What Is Induration on a TB Test?

Induration is the firm, raised or hardened area that develops under the skin after a tuberculin skin test.

It may feel:

Firm
Thickened
Slightly raised
Different from the surrounding skin

This is the area healthcare providers measure.

Redness around the test site is called erythema, and it is not included in the measurement.

Redness vs Induration: What Actually Makes a TB Test Positive?

This is one of the most important points when reading a TB skin test.

Redness

Redness can occur after the injection, but redness alone does not mean the test is positive.

A TB test may look:

Red
Pink
Slightly irritated

and still be negative.

Induration

Induration is the firm, raised area that healthcare professionals measure.

The test result is based on:

The size of the induration
Your TB risk factors
Your medical history

So, a red TB test is not automatically a positive TB test.

What Does a Positive TB Test Look Like After 48 Hours?

At around 48 hours, a positive TB skin test may show:

A firm bump
Noticeable induration
Slight elevation above surrounding skin
Redness around the injection site

However, redness is still not used to determine positivity.

A trained healthcare professional can begin evaluating the reaction during this time window.

What Does a Positive TB Test Look Like After 72 Hours?

At 72 hours, a positive test may still show:

A firm or hardened area
Measurable swelling
Raised induration
Redness in some cases

The test should generally be read within the recommended 48–72 hour window.

If it is not checked in time, your healthcare provider may recommend repeating the test.

What Size Bump Is Considered a Positive TB Test?

The size of the bump is interpreted differently depending on your risk category.

5 mm or More

An induration of 5 mm or more may be considered positive in certain high-risk individuals.

This can include people with:

HIV
Recent close contact with someone who has infectious TB
Certain chest X-ray findings suggesting previous TB
Significant immune suppression
Some transplant-related risk factors

10 mm or More

An induration of 10 mm or more may be considered positive in people with additional TB risk factors.

This may include:

People from countries where TB is more common
Certain healthcare workers
People living or working in some high-risk settings
People with particular medical conditions
Some children with specific risk factors

15 mm or More

An induration of 15 mm or more may be considered positive in people who do not have known TB risk factors.

This is why the exact same bump size may be interpreted differently in different people.

TB Test Bump Size Chart

TB Skin Test Measurement	Possible Interpretation
0–4 mm	Usually negative
5 mm or more	May be positive in high-risk individuals
10 mm or more	May be positive in people with certain risk factors
15 mm or more	May be positive in people without known TB risk factors

This chart is for general understanding only.

Your healthcare provider should interpret the actual result.

Does a Positive TB Test Always Have a Red Bump?

No.

A positive TB test does not have to be strongly red.

Some people may have:

Clear induration with little redness
Noticeable redness with only small induration
Both redness and induration

Only the hardened area is measured.

Does a Positive TB Test Always Have a Large Bump?

Not necessarily.

A bump as small as 5 mm may be considered positive in some high-risk people.

For someone with no known risk factors, a larger reaction may be required before the result is considered positive.

That is why bump size should never be interpreted without knowing the person's risk category.

Can a TB Test Be Positive Without Symptoms?

Yes.

A person can have a positive TB skin test and feel completely well.

This may indicate latent TB infection, meaning TB bacteria may be present in the body without causing active disease.

People with latent TB infection usually:

Do not feel sick
Do not have TB symptoms
Cannot spread TB to others
May be offered treatment to reduce the risk of developing active TB later

Does a Positive TB Test Mean Active Tuberculosis?

No.

A positive TB skin test does not automatically mean you have active TB disease.

It means your immune system has reacted to TB-related antigens.

Further testing is needed to determine whether you have:

Latent TB infection
Active TB disease
A false-positive result

Latent TB vs Active TB After a Positive Test

Latent TB Infection

With latent TB:

You usually have no symptoms
You do not spread TB to other people
Your chest X-ray may be normal
Treatment may be recommended to reduce future risk

Active TB Disease

Active TB can cause symptoms such as:

Persistent cough
Fever
Night sweats
Fatigue
Weight loss
Chest pain
Coughing up blood

Active TB requires medical treatment and may be contagious depending on where the infection is located.

What Happens After a Positive TB Test?

If your TB skin test is positive, your healthcare provider may recommend additional evaluation.

This may include:

Medical history
Physical examination
Chest X-ray
TB blood test
Sputum testing if active pulmonary TB is suspected

The goal is to determine whether TB infection is latent or active.

Can a Positive TB Skin Test Be False Positive?

Yes.

False-positive TB skin test results can occur.

Possible causes include:

Previous BCG vaccination
Exposure to certain non-tuberculosis mycobacteria
Incorrect test administration
Incorrect interpretation

This is one reason a TB blood test may sometimes be preferred.

Can the BCG Vaccine Cause a Positive TB Skin Test?

Yes, BCG vaccination can affect TB skin test results in some people.

The BCG vaccine is commonly used in countries where tuberculosis is more prevalent.

If you have received BCG, a healthcare provider may recommend a TB blood test because blood tests are generally not affected by BCG vaccination.

TB Skin Test vs TB Blood Test

Two common types of testing are used to detect TB infection.

TB Skin Test

Also known as:

TST
PPD test
Mantoux test

Advantages:

Widely available
Commonly used
Relatively simple

Limitations:

Requires a second visit
Must be read within 48–72 hours
Can be affected by BCG vaccination

TB Blood Test

Also called an interferon-gamma release assay (IGRA).

Examples include:

QuantiFERON-TB Gold
T-SPOT.TB

Advantages:

Requires one blood draw
Does not require a return visit for skin measurement
Is generally preferred for some people who received BCG

How Is a Positive TB Skin Test Measured?

A healthcare professional usually:

Examines the injection site
Feels the skin for induration
Identifies the edges of the hardened area
Measures the width across the forearm
Records the measurement in millimeters

The red area is not included.

Can You Read a TB Test at Home?

It is not recommended.

A TB skin test can be difficult to interpret accurately because induration may be easier to feel than see.

Healthcare professionals are trained to:

Identify the hardened area
Measure it correctly
Apply the correct cutoff based on your risk factors

Self-reading may lead to inaccurate conclusions.

What If My TB Test Is Red but Has No Hard Bump?

Redness without measurable induration is not usually considered a positive result by itself.

The important feature is the hardened area.

If you are unsure about the reaction, have it checked by the healthcare provider who administered the test.

What If My TB Test Has a Hard Bump but No Redness?

A hard bump can still be important even if the skin is not very red.

The healthcare provider measures the induration and compares it with the appropriate cutoff.

Redness is not required for a positive result.

What If the TB Test Bump Keeps Getting Bigger?

A growing or changing reaction should be evaluated by a healthcare professional.

The test should be officially read during the 48–72 hour window.

Do not try to diagnose the result yourself based on appearance alone.

Positive TB Test vs Negative TB Test

Feature	Positive TB Test	Negative TB Test
Induration	Meets the cutoff for the person's risk group	Below the cutoff
Redness	May or may not be present	May or may not be present
Raised area	Usually measurable	None or too small to meet criteria
Meaning	TB infection may be present	TB infection is less likely
Next step	Further evaluation often needed	Usually no further action unless symptoms or exposure exist

Positive TB Test Pictures: Can Images Confirm the Result?

Many people search for positive TB test pictures to compare their reaction with online examples.

Pictures can help show what induration may look like, but they cannot confirm whether a test is positive.

Why?

Skin tone differs
Lighting changes appearance
Induration may be easier to feel than see
Risk-based cutoffs differ
Redness does not equal positivity

A professional measurement is more reliable than comparing your arm with a photo.

When Should You Seek Medical Advice After a Positive Test?

Contact a healthcare provider if your TB test is positive or if you have TB symptoms such as:

Cough lasting several weeks
Fever
Night sweats
Unexplained weight loss
Persistent fatigue
Chest pain
Coughing up blood

You should also seek medical advice if you recently had close contact with someone diagnosed with active TB.

Frequently Asked Questions

What does a positive TB test look like?

A positive TB skin test usually has a firm, raised or hardened area called induration at the injection site. The size of the induration determines whether the result is positive.

How big is a positive TB test bump?

A reaction may be considered positive at 5 mm, 10 mm or 15 mm depending on the person's TB risk factors.

Is redness a sign of a positive TB test?

Redness alone does not make a TB test positive. Healthcare professionals measure induration, not redness.

What does a positive TB test look like after 48 hours?

A positive test may show firm, raised induration at 48 hours. The reaction should be measured by a healthcare professional.

What does a positive TB test look like after 72 hours?

At 72 hours, a positive result may still show measurable hardened swelling. The test is generally read during the 48–72 hour window.

Can a TB test be positive without symptoms?

Yes. Many people with latent TB infection feel well and do not have symptoms.

Does a positive TB test mean active TB?

No. A positive test indicates possible TB infection, but further testing is required to determine whether the infection is latent or active.

Can the BCG vaccine cause a positive skin test?

Yes. Previous BCG vaccination may affect TB skin test results in some people.

Can a positive TB test be wrong?

Yes. False-positive and false-negative results can occur.

What happens after a positive TB skin test?

A healthcare provider may recommend a chest X-ray, medical evaluation and additional testing to check for active TB disease.

Final Thoughts

If you are asking what does a TB positive test look like, the clearest answer is this:

A positive TB skin test usually has a firm, raised or hardened area called induration at the injection site. The result is based on the size of that induration—not the amount of redness.

Depending on your risk factors, a reaction of 5 mm, 10 mm or 15 mm or more may be considered positive.

A positive skin test does not automatically mean active tuberculosis. It means further medical evaluation may be needed to determine whether you have latent TB infection, active TB disease or another explanation for the reaction.

Always have a TB skin test read by a trained healthcare professional within the recommended 48–72 hour window.

This article is for general educational purposes and is not a substitute for professional medical diagnosis or treatment.

This guide is general education and is not medical, legal or career advice. Entry rules, fees, training lengths and treatment guidance change, so confirm them with the official body in your country or with your own doctor or pharmacist. Read our disclaimer. Found an error? Tell us.