A TB skin test involves injecting a tiny amount of protein from tuberculosis bacteria just under the skin of your forearm, then checking the spot 48 to 72 hours later for a raised bump. The size of that bump, combined with your personal risk factors, determines whether the result counts as positive. The test itself takes only a few minutes, but the full process spans two to three days because you must return for a trained healthcare worker to read the result.
How the Injection Works
The test uses a substance called tuberculin PPD, a purified protein that triggers an immune response if your body has previously encountered TB bacteria. A healthcare worker injects exactly 0.1 ml of this solution into the inner surface of your forearm, about 2 to 4 inches below the elbow joint. The injection goes into the top layer of skin (not into muscle or a vein), using a small syringe with a very fine 27-gauge needle angled at just 5 to 15 degrees.
The needle goes in bevel-side up so the opening faces away from the skin’s surface. When done correctly, a small, pale bump called a wheal forms immediately at the injection site, typically 6 to 10 mm across. That wheal is a sign the fluid landed in the right layer of skin. If no wheal appears, the injection needs to be repeated at a spot at least 2 inches away from the first attempt. The healthcare worker will choose an area on your forearm that’s free of scars, sores, or visible veins, since those can make the result harder to read later.
What to Do After the Injection
The small bump from the injection usually fades within minutes. In the two to three days before your reading appointment, leave the area alone. Don’t cover it with a bandage, apply lotion, or scratch it, since any of those can affect your results. You can wash the area gently with water, but avoid wiping or scrubbing. If itching bothers you, place an ice cube or a cold cloth on the spot.
The 48-to-72-Hour Reading Window
You’ll need to return to your healthcare provider between 48 and 72 hours after the injection. This window is strict. A trained healthcare worker reads the result by feeling the injection site for induration, which is a firm, raised area under the skin. They measure the width of that raised area in millimeters, using a ruler placed across your forearm.
The key detail: redness alone doesn’t count. Your skin may look red or slightly irritated around the site, but only the firm, raised portion matters for the measurement. If you don’t return within 72 hours, the test is no longer valid and you’ll need to start over with a new injection.
What the Measurements Mean
A TB skin test isn’t simply “positive” or “negative” based on one universal number. The threshold depends on your individual risk factors. Three cutoff points exist:
- 5 mm or more is positive for people at the highest risk: those with HIV, anyone who has recently been in close contact with someone who has active TB, people with old healed TB visible on a chest X-ray, organ transplant recipients, and anyone taking immune-suppressing medications or long-term corticosteroids.
- 10 mm or more is positive for people with moderate risk: recent arrivals (within five years) from countries where TB is common, people who inject drugs, residents and employees of prisons, nursing homes, hospitals, or homeless shelters, lab workers who handle TB samples, people with diabetes or chronic kidney disease, and children under four years old.
- 15 mm or more is positive for people with no known risk factors. A reaction this large is unlikely to be caused by a prior BCG vaccination or exposure to related environmental bacteria.
This tiered system exists because the test isn’t perfectly specific. People who received the BCG vaccine (common in many countries outside the U.S.) or who have been exposed to non-tuberculous bacteria in the environment can sometimes show a reaction even without TB infection. Using lower thresholds only for high-risk groups reduces the chance of missing a real infection in people who need treatment most.
What a Positive Result Means
A positive TB skin test means your immune system has encountered TB bacteria at some point. It does not necessarily mean you have active tuberculosis or that you’re contagious. Most people with a positive skin 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.
Two-Step Testing
Some workplaces and healthcare facilities require a two-step TB test. This is common for new employees at hospitals, nursing homes, and similar settings. The reason: in people who were infected with TB years ago, the immune response can fade over time, causing the first test to appear falsely negative. The first injection essentially “reminds” the immune system, and the second test, given one to three weeks later, produces a more accurate reading. If the first test is negative but the second comes back positive, it’s typically interpreted as a boosted response to an old infection rather than a new one. Two-step testing is only used for the initial baseline; after that, a single test is used for routine annual screening.
What the Experience Feels Like
The injection itself feels like a brief, sharp pinch. Because the needle only enters the very top layer of skin and the volume is tiny, most people find it less uncomfortable than a standard blood draw. You may notice a small itchy spot developing over the next day or two, which is normal. Some people see mild redness or slight swelling even without a positive result. The reading appointment is painless: the healthcare worker simply looks at and feels the injection site, measures any raised area, and records the result.