A TB skin test starts as a small, pale bump on the inner forearm right after injection, then either fades to nothing or develops a firm, raised area over the next two to three days. What your arm looks like at that follow-up visit is what determines your result. The appearance changes at each stage, and knowing what to expect helps you understand what’s normal and what counts as a positive reaction.
What the Test Looks Like Right After Injection
The standard TB skin test (called the Mantoux test) involves a small injection just under the surface of the skin on the inside of your forearm. Immediately after the needle goes in, a small, raised, pale bump appears, roughly 6 to 10 millimeters across. This bump, sometimes called a wheal, looks a bit like a mosquito bite. It’s caused by the small amount of testing fluid sitting just beneath the top layer of skin.
This initial bump usually fades within 20 to 30 minutes and is not your result. It simply confirms the injection was placed correctly. From there, you wait 48 to 72 hours before a healthcare provider reads the test.
Caring for the Site While You Wait
During the two- to three-day waiting period, you should avoid putting bandages, lotion, or any other products on the injection site. Don’t scratch or rub the area, as this can irritate the skin and affect results. You can wash gently with water, but don’t wipe or scrub. If it itches, placing an ice cube or cold cloth on the spot can help without interfering with the test.
What a Negative Result Looks Like
When you return to have your test read, a negative result is pretty unremarkable. The injection site may look completely normal, with no visible change at all. Some people develop mild redness around the area, which can be alarming, but redness alone does not count as a positive result. Healthcare providers are specifically trained to ignore redness and measure only induration, which is a firm, dense, raised area you can feel under the skin.
If there’s no induration, the result is recorded as 0 mm and considered negative. Even a small amount of firmness below certain size thresholds counts as negative, depending on your risk category.
What a Positive Result Looks Like
A positive TB skin test produces a noticeable area of firm swelling at the injection site. This induration feels hard and raised when you run a finger across it, distinctly different from the soft, flat skin around it. It often looks like a rounded bump, sometimes with redness spreading around it, though again, only the firm area matters for measurement.
The size of the induration determines whether the result is positive, and the threshold varies by risk level:
- 5 mm or larger is considered positive for people at highest risk, including those with weakened immune systems or recent close contact with someone who has active TB.
- 10 mm or larger is positive for people with moderate risk factors, such as recent immigrants from countries where TB is common, healthcare workers, and people living in group settings.
- 15 mm or larger is positive for anyone, even people with no known risk factors.
In some cases, a strongly positive reaction can produce blistering or small fluid-filled areas at the injection site. This is an intense immune response and is still measured the same way, by the diameter of the firm swelling. A reaction this strong typically indicates significant exposure to TB bacteria.
Redness vs. Firmness
The most common source of confusion is redness. Your arm might look red and irritated across a large area, which naturally makes people worry. But a healthcare provider will press on the skin to feel for the hard, raised bump underneath. A large red patch with no firm swelling beneath it is still a negative result. This is why you need a trained reader to interpret the test rather than judging it yourself at home.
The 48-to-72-Hour Reading Window
The test must be read between 48 and 72 hours after the injection. This window exists because the immune reaction peaks during this timeframe. If you miss the 72-hour mark, the reaction may begin to fade, making it unreliable. In most cases, a missed reading means you’ll need to repeat the test entirely.
How a TB Blood Test Differs
Not all TB tests happen on your skin. A TB blood test, known as an IGRA (interferon-gamma release assay), involves a standard blood draw from your arm. There’s nothing to look at on your skin afterward, no bump, no waiting period, and no return visit for a reading.
Instead, your results come back as a lab report. The report lists the overall result as positive, negative, or indeterminate, along with numerical values measuring how strongly your blood cells reacted to TB proteins. A positive blood test means your immune system recognizes TB bacteria, just like a positive skin test does. An indeterminate result means the test couldn’t produce a clear answer, often due to immune system issues, and may need to be repeated.
If You’ve Had the BCG Vaccine
The BCG vaccine, given in many countries outside the United States to protect against TB, can complicate skin test results. There is no reliable way to tell the difference between a skin test reaction caused by the BCG vaccine and one caused by actual TB infection. People who received BCG vaccination, especially those who received multiple doses, are more likely to show a false-positive skin test reaction.
This reactivity from BCG generally fades over time, but repeated skin testing can actually boost the reaction and keep it going longer. For this reason, TB blood tests are the preferred option for anyone who has received the BCG vaccine. Unlike the skin test, blood tests are unaffected by BCG vaccination and won’t produce false positives from it.
What a Positive Result Actually Means
A positive result on either test means your immune system has encountered TB bacteria at some point. It does not mean you have active TB disease or that you’re contagious. Most people with a positive test have what’s called latent TB infection, where the bacteria are present but dormant and not causing symptoms. A chest X-ray is typically the next step to check whether the infection is active. If the X-ray is clear and you have no symptoms like a persistent cough, fever, or unexplained weight loss, you likely have latent TB that can be treated to prevent it from ever becoming active.