A positive TB skin test means your immune system has encountered tuberculosis bacteria at some point. It does not mean you are sick with TB or contagious. In most cases, a positive result points to a latent infection, meaning the bacteria are present in your body but inactive, causing no symptoms and posing no risk to people around you. The next step is figuring out whether the infection is latent or has progressed to active disease.
How the Test Works
A small amount of purified protein derived from TB bacteria is injected just under the skin of your forearm. If your immune system has previously encountered TB, specialized immune cells recognize the protein and trigger a localized reaction at the injection site. This reaction takes time to develop, which is why you need to return 48 to 72 hours later to have it read. If you miss that window, the test can’t be interpreted and you’ll need a new one.
A trained healthcare worker measures the raised, firm bump (called induration) at the injection site. Redness alone doesn’t count. Only the size of the firm, raised area matters, and it’s measured in millimeters.
What Counts as “Positive” Depends on Your Risk
There isn’t a single cutoff for a positive result. The threshold changes based on how likely you are to have been exposed to TB or to develop serious disease from it.
- 5 mm or more is positive for people with the highest risk: those living with HIV, anyone recently in close contact with a person who has active TB, organ transplant recipients, and people on immunosuppressive medications such as long-term corticosteroids or certain biologic drugs.
- 10 mm or more is positive for people with moderate risk. This includes people born in countries where TB is common (such as Mexico, the Philippines, Vietnam, India, China, Haiti, and Guatemala), people who live or work in shelters, nursing homes, or correctional facilities, children under 5, people with diabetes, severe kidney disease, or certain cancers, and those with low body weight.
- 15 mm or more is positive for people with no known risk factors for TB.
The lower thresholds for higher-risk groups exist because even a small reaction in someone with a weakened immune system is significant. Their body may not mount as strong a response, so a smaller bump still signals real exposure.
Latent TB Infection vs. Active TB Disease
This distinction is the most important thing to understand after a positive skin test. The vast majority of people with a positive result have latent TB infection. With latent TB, you carry the bacteria but they’re dormant. You feel completely fine, have no symptoms, and cannot pass TB to anyone else.
Active TB disease is different. People with active TB typically feel sick. Common symptoms include a persistent cough lasting three weeks or longer, chest pain, coughing up blood, fatigue, fever, night sweats, chills, and unexplained weight loss. Active TB can spread to others through the air when someone coughs or sneezes. Only about 5 to 10 percent of people with latent TB ever develop active disease, though the risk is higher for people with weakened immune systems.
Why a Positive Result Might Be Wrong
The BCG vaccine, widely given to children in many countries outside the United States, can trigger a positive TB skin test even in someone who has never been infected. There is no reliable way to tell whether a positive skin test was caused by BCG vaccination or by actual TB infection. People who received multiple BCG doses, a practice in some countries, tend to have stronger and longer-lasting skin test reactions. In children, the vaccine is most likely to cause a positive reaction within the first two years after vaccination.
If you were vaccinated with BCG and your skin test comes back positive, your provider will likely recommend a TB blood test (called an interferon gamma release assay, or IGRA). This blood test is not affected by prior BCG vaccination, so it gives a clearer answer about whether you’ve truly been infected.
False negatives are also possible. A weakened immune system from HIV, certain medications, severe illness, or even a recent viral infection can prevent your body from reacting to the test protein, making it look like you haven’t been exposed when you actually have.
What Happens After a Positive Result
A positive skin test is a starting point, not a diagnosis. The standard next step is a chest X-ray to look for signs of active TB in the lungs. If the X-ray looks normal and you have no symptoms, you’ll typically be diagnosed with latent TB infection. If the X-ray shows abnormalities, your provider will collect sputum samples (mucus you cough up) to test for active TB bacteria.
You will not need to repeat a TB skin test in the future. Once you’ve tested positive, the skin test will continue to react, so repeating it provides no useful information. Future monitoring, if needed, is done with chest X-rays or blood tests.
Treatment for Latent TB
Even though latent TB doesn’t make you sick, treating it dramatically reduces the chance it will ever become active. The CDC now recommends shorter treatment courses over the older regimens. The preferred options are a 3-month once-weekly combination regimen, a 4-month daily regimen, or a 3-month daily combination regimen. These shorter courses have largely replaced the older approach of taking a single daily medication for 6 to 9 months, though that remains an alternative when the shorter regimens aren’t an option.
Side effects vary by regimen but are generally manageable. Your provider will monitor your liver function during treatment since the medications can stress the liver, particularly in people who drink alcohol regularly or have preexisting liver conditions. Most people complete the course without significant problems, and once finished, the latent infection is considered treated.
Treatment for active TB disease is more intensive and lasts longer, typically 6 to 9 months with multiple medications. People with active TB are usually no longer contagious after a few weeks of treatment, though the full course must be completed to clear the infection.