The Mantoux test is a skin test used to check whether you’ve been exposed to tuberculosis (TB). A small amount of purified protein from TB bacteria is injected just under the skin of your forearm, and the injection site is checked 48 to 72 hours later for signs of a reaction. It’s one of the oldest and most widely used TB screening tools in the world, often required for employment in healthcare, school enrollment, or immigration.
How the Test Works
The solution injected during a Mantoux test is called purified protein derivative, or PPD. It’s made from proteins isolated from the bacterium that causes TB. PPD doesn’t contain any live bacteria, so it can’t give you tuberculosis. What it does is provoke your immune system. If your body has encountered TB bacteria before, either through infection or vaccination, specialized immune cells recognize those proteins and mount a localized inflammatory response at the injection site.
This is a delayed-type hypersensitivity reaction, meaning it doesn’t happen immediately. Over the next two to three days, immune cells migrate to the injection site, causing blood vessels to widen and fluid to accumulate in the tissue. The result is a firm, raised area of skin called an induration. The size of that induration tells a clinician whether your immune system has previously been sensitized to TB.
What Happens During the Test
A healthcare provider injects 0.1 ml of PPD solution into the inner surface of your forearm, about 5 to 10 centimeters (2 to 4 inches) below the elbow joint. The injection uses a small 27-gauge needle inserted bevel-up at a very shallow angle of 5 to 15 degrees, just deep enough to sit within the top layers of skin rather than beneath them. When placed correctly, the injection creates a small, pale bump called a wheal, typically 6 to 10 mm across. This wheal usually fades within minutes.
The whole process takes less than a minute and feels like a brief sting. You’ll then need to return to the same provider 48 to 72 hours later to have the site read. Reading it outside that window can give unreliable results, so timing your return visit matters.
Caring for the Injection Site
Between the injection and your reading appointment, keep the site clean and dry. Don’t scratch it, even if it itches. Avoid applying creams, lotions, or adhesive bandages over the area, as these can irritate the skin and make the result harder to interpret. It’s fine to wash your arm gently, but don’t scrub the spot.
How Results Are Read
When you return, the provider measures only the induration: the firm, raised area you can feel under the skin. Redness around the site (called erythema) is common but doesn’t count. The measurement that matters is the diameter of the hard bump, recorded in millimeters. Your provider will typically run a finger across the site to find the edges of any firmness and then measure across it with a small ruler.
A Mantoux result isn’t simply “positive” or “negative.” The threshold for a positive reading depends on your individual risk for TB, and three cutoff levels are used:
- 5 mm or more is considered positive for people at the highest risk. This includes people living with HIV, those with weakened immune systems, anyone who has been in close contact with someone with active TB, and people whose chest X-ray shows signs of old TB infection.
- 10 mm or more is considered positive for people with moderate risk factors. This group includes recent immigrants from countries where TB is common, healthcare workers, people who live or work in congregate settings like shelters or prisons, and children under four years old.
- 15 mm or more is considered positive for people with no known risk factors for TB.
This sliding scale exists because the consequences of missing a TB diagnosis are more serious for high-risk individuals, so the threshold is set lower to catch more cases.
What a Positive Result Means
A positive Mantoux test does not mean you have active tuberculosis. It means your immune system has been exposed to TB proteins at some point. You could have a latent TB infection, where the bacteria are present in your body but dormant and not causing symptoms or spreading to others. Or the reaction could be from a prior BCG vaccination, which is a TB vaccine given routinely in many countries outside the United States.
If your result is positive, the next step is usually a chest X-ray to look for signs of active disease in your lungs. You may also be asked about symptoms like a persistent cough, fever, night sweats, or unexplained weight loss. If the X-ray is clear and you have no symptoms, you likely have latent TB, which can be treated to prevent it from becoming active later.
Why Results Can Be Misleading
The Mantoux test has well-known limitations in both directions. False positives are common in people who received the BCG vaccine, particularly if they were vaccinated after infancy or received multiple doses. The proteins in the vaccine overlap enough with PPD to trigger a reaction even without true TB infection. This is a frequent source of confusion for people born in countries where BCG vaccination is standard.
False negatives also happen. People with severely weakened immune systems, whether from HIV, certain cancers, immunosuppressive medications, or severe malnutrition, may not mount a reaction even if they are infected with TB. This condition, called anergy, means the immune system is too suppressed to respond to the test. Recent viral infections, including measles, and even recent live-virus vaccinations can temporarily reduce the skin’s ability to react. Very young children and the elderly are also more likely to have a muted response.
Mantoux vs. TB Blood Tests
An alternative to the Mantoux test is a blood test called an interferon-gamma release assay, or IGRA. Instead of injecting something into your skin, a blood sample is drawn and tested in a lab for how your immune cells respond to TB-specific proteins. The main advantage of a blood test is that it requires only one visit and is not affected by prior BCG vaccination, making it more reliable for people who were vaccinated as children.
Both tests detect immune sensitization to TB, not active disease, so either way a positive result leads to the same follow-up steps. The CDC considers both the Mantoux test and IGRA blood tests acceptable for TB screening. In practice, the choice often comes down to logistics: the Mantoux test is cheaper and doesn’t require a lab, while blood tests are more convenient for people who can’t easily return for a 48-to-72-hour reading.
Two-Step Testing
If you’re starting a job that requires regular TB screening, such as in a hospital or long-term care facility, you may be asked to do a two-step Mantoux test. This involves getting a first test, having it read, and then repeating the entire process one to three weeks later. The reason is that in some people who were infected with TB years ago, the immune response has faded enough that the first test comes back negative. The first injection essentially “reminds” the immune system, so the second test gives a more accurate baseline. Without this step, a future routine test might look like a new positive when it’s actually a boosted old reaction.