A TB skin test is a simple screening tool that checks whether your body has been exposed to the bacteria that cause tuberculosis. A small amount of protein from the TB organism is injected just under the skin of your forearm, and a healthcare worker reads the result 48 to 72 hours later by measuring any firm, raised bump that forms at the injection site. It doesn’t test whether you have active TB disease, only whether your immune system recognizes the bacteria.
How the Test Works
The test uses a purified protein extracted from TB bacteria. A healthcare worker injects 0.1 ml of this solution into the inner surface of your forearm, about 2 to 4 inches below the elbow. The needle goes in at a very shallow angle, just 5 to 15 degrees, so it sits right beneath the top layer of skin. When the injection is done correctly, a small, pale, firm bump (called a wheal) immediately appears at the site. This bump is normal and fades within minutes.
What happens next is an immune response. If your body has encountered TB bacteria before, either through infection or vaccination, your immune system’s T cells recognize the injected protein and mount a reaction at the site. These T cells release signaling molecules that trigger inflammation, producing a firm, raised area you can feel under the skin. This type of immune reaction is called a delayed-type hypersensitivity response because it takes time to develop, typically peaking between 48 and 72 hours after the injection. That’s why you need to return to have the test read within that window.
What Happens at Your Appointment
The test itself takes only a few minutes. You’ll rest your forearm palm-side up on a flat surface while the healthcare worker cleans a spot on your inner forearm with an alcohol swab. They’ll choose an area free of scars, sores, or anything else that could make the result hard to read. The injection feels like a brief, mild sting. You don’t need to bandage the site, and you can go about your normal activities immediately afterward.
You must return 48 to 72 hours later to have the result read. This is the part many people don’t realize: you cannot read the test yourself, and a healthcare worker trained in TB skin test interpretation needs to examine it. If you miss the 72-hour window, the test is invalid and you’ll need to schedule a new one. A second test can be placed as soon as possible after a missed reading.
How Results Are Measured
When you return for your reading, the healthcare worker will feel the injection site with their fingertips. They’re looking for induration, which is the firm, raised area beneath the skin. This is different from redness. Your skin may look red or slightly irritated around the injection site, but redness alone doesn’t count. Only the width of the firm bump matters, measured in millimeters across the forearm.
The threshold for a “positive” result isn’t the same for everyone. It depends on your risk factors:
- 5 mm or more is considered positive for people at highest risk, including those with HIV, people who have had recent close contact with someone with active TB, and people with weakened immune systems.
- 10 mm or more is considered positive for people with moderate risk factors, such as healthcare workers, people who have recently moved from countries where TB is common, and people who live or work in group settings like shelters or prisons.
- 15 mm or more is considered positive for people with no known risk factors.
A result of 0 mm, meaning no detectable bump, is recorded as negative. It’s worth knowing that the result is always documented in millimeters, not simply as “positive” or “negative,” because interpretation can change depending on your individual circumstances.
What a Positive Result Means
A positive TB skin test means your immune system recognizes TB bacteria. It does not mean you have active tuberculosis or that you’re contagious. Most people with a positive skin test have what’s called latent TB infection: the bacteria are in the body but dormant, causing no symptoms and posing no risk to others. Only a small percentage of people with latent TB ever develop active disease.
If your skin test is positive, you’ll typically need a chest X-ray to check for signs of active TB in the lungs. If the X-ray is normal and you have no symptoms like a persistent cough, fever, night sweats, or unexplained weight loss, you likely have latent TB. Your provider may recommend treatment to prevent the infection from becoming active in the future, which usually involves taking medication for several months.
False Positives and the BCG Vaccine
One well-known limitation of the TB skin test involves the BCG vaccine, which is widely given in countries where TB is common. Because the vaccine contains a weakened relative of TB bacteria, it can prime your immune system to react to the skin test even if you’ve never had an actual TB infection. This cross-reactivity is especially likely if you received BCG as an infant and were tested within the first several years, though it can persist longer in some people.
For this reason, the CDC recommends TB blood tests (known as IGRAs) as the preferred testing method for people who have received the BCG vaccine. Blood tests measure your immune response in a lab rather than on your skin, and they’re much less likely to react to prior BCG vaccination.
False Negatives
The test can also miss a real infection. People with severely weakened immune systems, including those with advanced HIV, may not mount enough of an immune response to produce a detectable bump. This is sometimes called anergy. Recent viral infections, certain medications that suppress the immune system, and even very recent TB exposure (within the past 8 to 10 weeks) can also lead to a false negative. The immune system simply hasn’t had enough time or capacity to recognize and respond to the protein.
TB Blood Tests as an Alternative
TB blood tests, or interferon-gamma release assays, offer an alternative to the skin test. Instead of injecting protein under the skin and waiting for a visible reaction, a blood sample is drawn and sent to a lab where it’s mixed with TB-specific proteins. The lab measures how strongly your immune cells respond.
Blood tests have two practical advantages. First, they require only one visit since there’s no 48-to-72-hour reading window. This makes them a better option for people who might not return for a follow-up appointment. Second, they’re less affected by prior BCG vaccination, making them more reliable for anyone who grew up in a country where that vaccine is routine. The CDC encourages healthcare providers to use blood tests when either of these situations applies.
Repeat Testing
There’s no health risk from getting multiple TB skin tests. You can be tested repeatedly over the years, which is common for healthcare workers and others in jobs that require regular screening. However, repeated testing can sometimes cause a phenomenon called boosting, where a weak immune memory from a very old TB exposure gets “reminded” by the test itself, leading to a stronger reaction on subsequent tests. This can look like a new positive result when it’s actually a boosted response to a very old exposure. Two-step testing, where a second skin test is placed one to three weeks after the first, is sometimes used for new healthcare workers to establish an accurate baseline and account for this effect.
If you’ve ever had a severe reaction to a TB skin test, such as blistering or ulceration at the injection site, repeat placement is generally not recommended. A blood test would be used instead.