The difference between a positive and negative TB test bump comes down to size and your risk factors. A positive TB test shows a firm, red bump that’s 5mm or larger (depending on your health), while a negative test has no bump or a very small one under 5mm.

What Does a TB Test Bump Look Like?

A TB test bump appears at the spot where the doctor injected the test fluid into your arm. According to the Centers for Disease Control and Prevention, if you’ve ever been exposed to the TB bacteria, your skin will react by developing a firm, red bump at the site within three days.

Positive TB Test Bump

A positive TB test bump means you have TB germs in your body. The Cleveland Clinic explains that your provider will measure the size of the firmness of the bump and consider your risk factors.

Size Requirements:

• High-risk people: 5mm or larger
• Medium-risk people: 10mm or larger
• Low-risk people: 15mm or larger

The bump feels firm when touched and stands out from the surrounding skin. The size, not the color, determines your results.

Negative TB Test Bump

A negative TB test means you likely don’t have TB bacteria in your body. The test is “negative” if there is no bump or only a very small bump (less than 5mm) at the injection site.

How Results Are Measured

Your healthcare provider must check your arm 48-72 hours after the TB skin test. They measure only the firm, raised part (induration) – not the red area around it. Healthcare professionals measure the induration across your forearm, perpendicular to the axis between your hand and elbow.

Risk Factors That Determine Your Results

High-Risk Groups (5mm = Positive)

You’re considered high-risk if you have:

• HIV or compromised immune system
• Recent contact with someone who has active TB
• Chest X-ray showing old TB scars
• Taking immune-suppressing medications

Medium-Risk Groups (10mm = Positive)

You’re medium-risk if you:

• Were born outside the United States
• Use injection drugs
• Live or work in high-risk places (hospitals, nursing homes, shelters)
• Have diabetes, kidney problems, or chronic conditions

Low-Risk Groups (15mm = Positive)

Low-risk people are healthy adults and children over 4 with no TB risk factors. They need a 15mm bump for a positive result.

False Results Can Happen

TB tests aren’t always accurate. Research published by the National Institutes of Health shows that false results can occur due to various factors.

False Positive Causes:

• Previous BCG vaccination
• Infection with other bacteria types
• Incorrect test administration

False Negative Causes:

• Recent TB infection (within 8-10 weeks)
• Weak immune system
• Very young or old age
• Recent live virus vaccines

What Happens Next

If Your Test Is Negative

A negative result usually means you don’t have TB bacteria. You might need more testing if you have TB symptoms or were recently exposed to someone with TB.

If Your Test Is Positive

A positive result means you have TB bacteria, but most people have latent TB (sleeping bacteria that can’t spread). Your doctor will order:

• Chest X-ray
• Sputum test
• Physical exam

Treatment

According to Mayo Clinic, treatment for latent TB infection is 90% effective at preventing active disease. Treatment usually involves taking medicine for 3-9 months.

Care After Your Test

Keep the injection site clean and dry. Don’t cover it with bandages or scratch it. If it itches, put a cold cloth on it.

When to Get Tested

You should get a TB test if you:

• Work in healthcare or crowded places
• Have been around someone with TB
• Come from a country with high TB rates
• Have HIV or immune problems

Yorktown Health offers TB test administration in Vernon Hills with expert interpretation of results.

Final Thoughts

Understanding TB test bumps is straightforward: size and your risk factors determine if results are positive or negative. Always have a trained healthcare provider read your results within 48-72 hours.

If you need a TB test, contact Yorktown Health in Vernon Hills. Our team provides clear explanations and expert care to help you understand your health results.