How to Tell if Your TB Test Bump Is Positive or Negative

What Is a TB Test?

A TB test (short for tuberculosis) checks whether your body has been exposed to the bacteria that cause this disease. When people say “TB test,” they usually mean the tuberculin skin test (TST). There are two types of TB tests: a skin test and a blood test. 

During the test, a small amount of fluid called tuberculin is injected just under the skin of your forearm. This creates a small raised area right away. That’s normal.

What matters is what the bump looks and feels like 48 to 72 hours later. If your body has seen TB bacteria before, your immune system reacts, creating a firmer bump.

How Can the TB Test Bump Be Interpreted?

Negative TB Test Bump

A negative test means there was little or no reaction to the test fluid.

It often looks like:

• No bump at all, or one smaller than 5 millimeters
• Soft or flat to the touch
• A little red or itchy, but not firm

This usually means you haven’t been exposed to TB. But in some cases, more testing might be needed if you have symptoms or recent exposure.

Positive TB Test Bump

A positive result means your skin reacted more strongly. That suggests you’ve been exposed to TB bacteria, but it doesn't mean you're sick or contagious.

The bump is:

• Firm and raised
• Doesn’t flatten when pressed
• Usually compared to a small pea or pencil eraser

What counts as “positive” depends on your health history:

• 5mm or more: high-risk people
• 10mm or more: moderate-risk groups
• 15mm or more: low-risk people with no known TB risk

Positive vs Negative TB Test Bump: Quick Comparison

Here’s a quick side-by-side look at positive vs negative results:

How Do Doctors Measure the Bump?

Doctors measure only the raised, firm part of the bump. Redness doesn’t matter.

• They measure across your forearm, not up and down
• They use a ruler for accuracy
• They measure 48 to 72 hours after the injection

Note: Do not try to measure it yourself. And don’t scratch or poke the bump; it can affect the skin and confuse the results.

What If You Missed the 72-Hour Window?

Life happens. If you forgot to go back for your test reading, it doesn’t mean you're stuck.

Here's what you can do:

• Let your provider know it’s not too late to plan a repeat test
• You’ll likely need to wait at least a week or more before getting tested again to avoid confusing results
• Don’t try to interpret the bump yourself after the 72-hour mark, it may no longer be accurate

If the bump faded and you’re asymptomatic, that’s often a good sign. Still, follow your doctor’s guidance.

Can TB Test Results Be Wrong?

Yes, TB skin tests can give false positives or false negatives.

Reasons for a false positive:

• You received the BCG vaccine as a child (common in other countries)
• You've been exposed to non-TB bacteria that trigger a similar reaction

Reasons for a false negative:

• Recent TB exposure (your immune system hasn’t reacted yet)
• Weakened immune system (due to age, illness, or medications)
• You recently got a live-virus vaccine (like MMR)

This is why doctors don’t rely on the skin test alone for a diagnosis. Your personal history matters.

What Happens If Your TB Test Is Positive?

A positive skin test means your body has been exposed to TB bacteria, but not necessarily that you’re sick or contagious.

Next steps usually include:

• Chest X-ray: to check for signs of active TB
• Sputum test: a sample of mucus from your lungs
• Physical exam: to look for TB symptoms

If you don’t have active TB, you might still have a latent TB infection. This means the bacteria are in your body but not causing illness. Inactive TB can turn active later, which is why doctors often recommend preventive treatment (like vaccinations).

Latent TB treatment is usually very effective and helps prevent serious illness down the road.

When Should You See a Doctor?

You should call your provider if:

• Your bump is firm and raised after 48-72 hours
• You missed your follow-up window
• You have TB symptoms like coughing, fever, or night sweats
• You’ve been in close contact with someone who has TB

Even if your test is negative, but you feel unwell or were recently exposed, it’s smart to follow up. TB can be managed and treated successfully, but only if it’s caught early.