A TB skin test is positive when the firm, raised bump on your forearm reaches a certain size, but the threshold depends on your personal risk factors. For most people with no known risk factors, the bump needs to measure at least 15 mm (about the width of a dime) to count as positive. For people with higher risk, a smaller reaction can be significant. Redness alone does not determine the result.
What You’re Actually Measuring
After a TB skin test (also called a Mantoux test), a healthcare provider injects a small amount of protein from TB bacteria just under the skin of your inner forearm. You then return 48 to 72 hours later to have the site read. The key measurement is the induration: the firm, hard, raised area you can feel when you run your finger across the injection site. This is different from erythema, which is just redness or discoloration around the area.
Many people see redness and assume their test is positive. It’s not that simple. The CDC specifically instructs that only the induration should be measured, not the redness. You could have a large red patch with no firm swelling underneath, and that would not be a positive result. Conversely, a small but distinctly hard bump in someone with HIV could be positive. The measurement is taken in millimeters across the widest point of the firm area.
The Three Size Thresholds
There isn’t a single number that makes a TB test positive for everyone. The cutoff shifts based on how likely you are to have been exposed to TB or how vulnerable you’d be if infected. There are three tiers.
5 mm or Larger
This is the lowest threshold, reserved for people at the highest risk of serious TB infection:
- People living with HIV
- Recent close contacts of someone with active TB
- Organ transplant recipients
- People on immunosuppressive medications, such as long-term corticosteroids or drugs that block inflammation
- People with old TB scarring visible on a chest X-ray
10 mm or Larger
This middle threshold applies to people with moderate risk:
- People born in countries where TB is common, including Mexico, the Philippines, Vietnam, India, China, Haiti, and Guatemala
- People who live or work in high-risk group settings like nursing homes, homeless shelters, or correctional facilities
- People with certain medical conditions that raise TB risk, including diabetes, severe kidney disease, certain cancers, and silicosis
- Children younger than 5
- People who misuse drugs or alcohol
- People with low body weight (below 90% of ideal body weight)
15 mm or Larger
If you have no known risk factors for TB, your test is only considered positive at 15 mm or more. This is also the threshold used for healthcare workers being screened at a new job who are otherwise low risk.
How a TB Blood Test Works Differently
If you had a blood test instead of a skin test, the interpretation is more straightforward. TB blood tests (called IGRAs) measure how your immune cells react to TB proteins in a lab. The result comes back as positive, negative, or indeterminate. There’s no size measurement for you to evaluate yourself, and no risk-based sliding scale. A positive blood test usually means TB infection.
Blood tests have one practical advantage: they aren’t affected by prior BCG vaccination. The BCG vaccine, given routinely in many countries outside the U.S., can sometimes cause a reaction on the skin test even without actual TB infection. If you were vaccinated with BCG as a child and now have a positive skin test, a blood test can help clarify whether the reaction reflects true TB infection or a vaccine-related response.
Positive Doesn’t Necessarily Mean You’re Sick
A positive TB test, whether skin or blood, tells you that TB bacteria have entered your body at some point. It does not tell you whether you have active TB disease. Most people with a positive result have what’s called latent TB infection. With latent TB, the bacteria are in your body but your immune system is keeping them in check. You feel completely fine, you have no symptoms, and you cannot spread TB to anyone else.
Active TB disease is different. People with active TB typically feel sick. Symptoms include a persistent cough lasting three weeks or longer, chest pain, coughing up blood, fatigue, unexplained weight loss, fever, and night sweats. Active TB can spread to others through the air when someone coughs or speaks. Both latent infection and active disease produce a positive screening test, which is why further evaluation is always the next step.
What Happens After a Positive Result
A positive screening test triggers a medical evaluation to determine whether you have latent infection or active disease. The first step is a chest X-ray, which can reveal lung abnormalities associated with active TB. Your provider will also review your medical history, ask about symptoms, and do a physical exam.
If the chest X-ray looks normal and you have no symptoms, you most likely have latent TB infection. Treatment for latent TB is shorter and simpler than treatment for active disease, and it significantly reduces the chance that the infection will progress to active TB later in life. About 5 to 10% of people with untreated latent TB will eventually develop active disease, so treatment matters even when you feel fine.
If the X-ray shows something abnormal or you have symptoms, further testing follows. The gold standard for confirming active TB disease is a bacteriologic examination, which involves collecting sputum samples (mucus you cough up from your lungs) and testing them in a lab to see if TB bacteria are actually growing. This can take several weeks for a final culture result, though faster molecular tests can provide preliminary answers sooner.
Can a TB Test Be Wrong?
False positives on the skin test can happen, particularly in people who received the BCG vaccine or who have been exposed to certain non-TB bacteria found in the environment. False negatives are also possible, especially in people with weakened immune systems whose bodies may not mount enough of a response to produce a measurable bump. Very recent infections can also be missed because the immune system needs two to eight weeks after exposure to develop a detectable reaction.
If your result seems inconsistent with your risk level or symptoms, a blood test can serve as a second opinion. Because the skin test and blood test work through different mechanisms, using one to verify the other can help resolve ambiguous cases.