A positive TB test means your immune system has encountered tuberculosis bacteria at some point. It does not mean you are sick or contagious. In most cases, a positive result points to latent TB infection, where the bacteria are alive in your body but inactive. The next step is additional testing to determine whether the infection is latent or has progressed to active TB disease.
Latent TB vs. Active TB Disease
This is the most important distinction after a positive test. Latent TB infection means a small number of TB bacteria are living in your body in a dormant state. You feel completely normal, have no symptoms, and cannot spread TB to anyone. You could carry latent TB for years, even a lifetime, without it ever causing problems.
Active TB disease is different. The bacteria are multiplying and attacking tissue, usually in the lungs. People with active TB typically feel sick, with symptoms like a persistent cough lasting three weeks or longer, chest pain, coughing up blood, fever, night sweats, fatigue, and unexplained weight loss. Active TB can be contagious, spreading through the air when someone coughs, speaks, or sings.
The critical point: a positive test alone cannot tell you which one you have. That requires follow-up testing, starting with a chest X-ray.
Two Types of TB Tests
There are two ways to test for TB, and both detect your immune system’s response to TB bacteria rather than the bacteria themselves.
The TB skin test (also called the Mantoux test or TST) involves a small injection of a protein derived from TB bacteria into the skin of your forearm. You return to your provider 48 to 72 hours later to have the injection site checked. They measure any raised, firm bump (called induration) in millimeters. The size required for a “positive” reading depends on your risk factors, which is explained in the next section.
The TB blood test (called an IGRA, or interferon-gamma release assay) is a single blood draw analyzed in a lab. Results come back as positive, negative, or indeterminate. A positive result means TB infection is likely. An indeterminate result means the test couldn’t provide a clear answer, and you may need to repeat it or try the skin test instead.
One Key Difference Between the Two Tests
If you received the BCG vaccine, which is given routinely in many countries outside the United States, the skin test can react to that vaccine and produce a false positive. The blood test is not affected by prior BCG vaccination, making it the more reliable option for people who were vaccinated. This is one reason blood tests have become increasingly preferred.
What Counts as “Positive” on a Skin Test
A skin test isn’t simply positive or negative. The threshold depends on how likely you are to have been exposed to TB and how vulnerable you are to developing active disease. There are three cutoff levels.
5 millimeters or more is positive for people at the highest risk: those with HIV, anyone who has recently been in close contact with someone with active TB, people with organ transplants, and those on medications that suppress the immune system (such as long-term corticosteroids or certain biologic drugs).
10 millimeters or more is positive for people with moderate risk factors. This includes people born in countries where TB is common (parts of Asia, Africa, and Latin America), people who live or work in nursing homes, homeless shelters, or correctional facilities, those with diabetes, severe kidney disease, or certain cancers, children under five, and people with low body weight.
15 millimeters or more is positive for people with no known risk factors for TB. If you’re an otherwise healthy adult with no particular exposure history, your bump needs to be at least 15 millimeters to be considered a true positive.
These tiered cutoffs exist because the consequences of missing TB are far more serious in someone who is immunocompromised than in someone with a healthy immune system. A lower threshold casts a wider net for the people who need it most.
What Happens After a Positive Test
A positive TB test triggers a medical evaluation with up to five components: your medical history, a physical exam, the TB test result itself, a chest X-ray, and in some cases a sputum sample (where you cough up material from your lungs for lab analysis).
The chest X-ray is the key next step. It can reveal signs of active TB disease in the lungs, such as areas of infection or unusual nodules. If your chest X-ray looks normal and you have no symptoms, you almost certainly have latent TB infection rather than active disease.
If the X-ray does show abnormalities, your provider will collect sputum samples to check for active TB bacteria under a microscope and through culture testing. This confirms whether you have active disease and helps determine which medications will work best.
Treatment for Latent TB Infection
Even though latent TB doesn’t make you sick right now, treatment is generally recommended because there’s a risk it could reactivate into active disease later. That risk is highest in the first two years after infection and in people with weakened immune systems.
Treatment for latent TB has gotten significantly shorter in recent years. The CDC now recommends shorter regimens lasting three to four months over the older approach of taking a single medication daily for six to nine months. These shorter courses are easier to complete and work just as well. Treatment for latent TB is preventive: it kills the dormant bacteria before they have a chance to wake up.
Treatment for active TB disease is a longer and more intensive process, typically involving multiple medications taken for several months. The exact duration depends on how the bacteria respond to treatment.
Can You Spread TB With a Positive Test?
If you have latent TB infection, you cannot spread TB bacteria to others. This is worth repeating because it’s the biggest source of anxiety for people who just got a positive result. Latent TB bacteria are walled off by your immune system. They are not active, not replicating in a way that reaches your airways, and not transmissible.
Only people with active TB disease in the lungs or throat can potentially spread the bacteria, and even then, it typically requires prolonged close contact. Casual interactions like brief encounters in public are low risk.
Can a Positive Test Be Wrong?
False positives do happen, especially with the skin test. The most common cause is prior BCG vaccination, which can trigger a reaction that looks identical to a true positive. Infection with other, non-tuberculosis species of mycobacteria found in the environment can also cause a false positive on the skin test.
If there’s any doubt about a skin test result, a TB blood test can help clarify things. Because the blood test uses antigens specific to the TB bacterium, it’s far less likely to cross-react with BCG or environmental bacteria. Your provider may order one to confirm or rule out a skin test finding, particularly if you were vaccinated with BCG as a child.
False negatives are also possible with both tests, especially in people with severely weakened immune systems whose bodies may not mount enough of an immune response to register on the test.