A Mantoux test is a skin test used to determine whether you’ve been exposed to tuberculosis (TB) bacteria. A small amount of protein derived from TB bacteria is injected just under the skin of your forearm, and the injection site is checked two to three days later for signs of an immune reaction. It’s one of the most widely used screening tools for TB infection worldwide.
How the Test Works
The test uses a substance called purified protein derivative, or PPD, which is made from proteins produced by the bacteria that cause tuberculosis. PPD itself cannot give you TB. It simply acts as a signal. If your immune system has encountered TB bacteria before, whether from an active infection or a past exposure, certain immune cells (T cells) will recognize the PPD and mount a response at the injection site. This response is a type of allergic reaction that builds slowly over 48 to 72 hours, which is why you can’t get your results the same day.
If your immune system has never encountered TB, the PPD goes unrecognized and nothing significant happens at the injection site.
What Happens During the Test
A healthcare worker injects 0.1 ml of PPD solution into the top layer of skin on the inside of your forearm, using a very small needle (27-gauge) inserted at a shallow angle of 5 to 15 degrees. The needle goes just deep enough that you can see the tip beneath the skin surface. When the injection is done correctly, a small, firm, pale bump called a wheal forms immediately at the site. This bump is normal and typically fades within minutes to hours.
The injection feels like a brief sting or pinch. The whole process takes under a minute.
Reading the Results
You’ll need to return to your healthcare provider between 48 and 72 hours after the injection to have the site read. This timing matters: reading it too early or too late can give inaccurate results. If you miss the window, the test generally needs to be repeated.
The provider measures any induration at the injection site, which is a firm, raised area of thickened skin you can feel beneath the surface. What they are not measuring is redness. Redness (erythema) around the site is common but doesn’t count. Only the diameter of the firm, thickened area matters, measured in millimeters with a ruler. If there’s no induration at all, the result is recorded as 0 mm.
Results are always recorded as a measurement in millimeters, not simply as “positive” or “negative.” That’s because the threshold for a significant result depends on your personal risk factors.
What the Millimeter Cutoffs Mean
Not everyone is held to the same standard when interpreting a Mantoux result. The measurement that counts as significant shifts depending on how likely you are to have been exposed to TB or how vulnerable you’d be if infected.
- 5 mm or more is considered significant for people at highest risk: those with HIV, people who have had recent close contact with someone with active TB, organ transplant recipients, and anyone on medications that suppress the immune system.
- 10 mm or more is considered significant for people with moderate risk, including recent immigrants from countries where TB is common, healthcare workers, people who use injectable drugs, and residents or staff of congregate settings like prisons or shelters.
- 15 mm or more is considered significant for people with no known risk factors for TB.
These thresholds exist because a smaller reaction in a high-risk person is more likely to represent real TB exposure, while a low-risk person needs a larger reaction before the result is meaningful.
What a Positive Result Means
A positive Mantoux test means your immune system has been sensitized to TB proteins. It does not mean you have active tuberculosis. Most people with a positive skin test have latent TB infection, meaning the bacteria are present in the body but dormant. They aren’t sick and can’t spread TB to others.
After a positive result, the next step is typically a chest X-ray to look for signs of active TB disease in the lungs. Your provider will also take a medical history and do a physical exam. If the chest X-ray looks abnormal or you have symptoms like a persistent cough, fever, night sweats, or unexplained weight loss, further testing follows. That can include sputum samples (coughed-up mucus) analyzed in a lab for TB bacteria.
A TB blood test, known as an interferon-gamma release assay (IGRA), is sometimes used as a follow-up to help confirm or clarify a skin test result.
Factors That Can Affect Accuracy
The Mantoux test is reliable, but several things can skew results in either direction.
The BCG vaccine, which is given routinely in many countries to protect against TB, can cause a false-positive result. The vaccine contains a related strain of bacteria, and your immune system may react to the PPD even though you don’t have a TB infection. This is one reason TB blood tests are sometimes preferred for people who received the BCG vaccine, since blood tests are not affected by prior vaccination.
False-negative results, where you have TB but the test misses it, can happen if your immune system is weakened. Conditions like HIV, certain cancers, or medications that suppress immunity can prevent your body from mounting the expected skin reaction. Recent viral infections, severe malnutrition, and even very recent TB infection (within the past 8 to 10 weeks) can also blunt the response, because the immune system hasn’t had enough time to develop sensitivity to TB proteins.
Two-Step Testing
Some people, particularly healthcare workers being tested for the first time as part of employment screening, undergo a two-step Mantoux test. The reason is a phenomenon called the booster effect. In someone who was exposed to TB years ago, the immune memory may have faded enough that the first test comes back negative. But that first injection can “wake up” the immune system’s memory, so a second test done one to three weeks later produces the true positive result.
Without two-step testing, that person might test negative on their initial screening and then test positive on a routine follow-up a year later, making it look like a new infection when it was actually an old one. Two-step testing establishes an accurate baseline so future results can be compared meaningfully.
Mantoux Test vs. TB Blood Test
The main alternative to the Mantoux test is a TB blood test (IGRA), which requires a single blood draw and doesn’t need a follow-up visit for reading. Blood tests have some practical advantages: they aren’t affected by BCG vaccination, they require only one visit, and they avoid the subjectivity of measuring skin induration. However, they cost more, require a laboratory to process, and aren’t always available in every setting.
Both tests detect immune sensitization to TB, not active disease. Neither can distinguish between latent and active TB on its own. The choice between them often comes down to your vaccination history, risk profile, and what’s available at your testing site.