A TB screening is a test that checks whether you’ve been infected with the bacteria that cause tuberculosis. It doesn’t tell you whether you’re sick with active TB. Instead, it detects whether TB bacteria are living in your body, often in a dormant state where you have no symptoms and can’t spread the infection to anyone else. Most people encounter TB screening as a requirement for a new job, school enrollment, or healthcare work.
How TB Screening Works
There are two types of TB screening tests used in the United States: a skin test and a blood test. Both measure your immune system’s response to TB proteins rather than looking for the bacteria directly. If your immune system recognizes TB proteins, it means you’ve been exposed to TB at some point.
The TB skin test (sometimes called a PPD or Mantoux test) involves a small injection just under the skin on the inside of your forearm. A healthcare provider injects a tiny amount of purified TB protein, which raises a small bump about 6 to 10 mm across. You then need to return 48 to 72 hours later so a trained reader can measure your skin’s reaction. They’re looking for a firm, raised area at the injection site, not just redness. The size of that raised area, combined with your personal risk factors, determines whether the result counts as positive.
The TB blood test (called an IGRA) requires a single blood draw with no return visit. A lab mixes your blood sample with proteins that mimic TB bacteria and measures whether your white blood cells release a specific signal in response. Two versions are FDA-approved: QuantiFERON-TB Gold Plus and T-SPOT.TB. Results typically come back within a few days.
Skin Test vs. Blood Test
Both tests serve the same purpose, but they have practical differences that matter depending on your situation. The skin test is cheaper and widely available, but it requires two visits and a trained person to read the result. The blood test needs only one visit and is processed in a lab, removing the subjectivity of reading a skin reaction.
One important distinction: if you’ve ever received the BCG vaccine, which is a TB vaccine given routinely in many countries across Asia, Africa, and Latin America, a skin test may give a false-positive result. There’s no reliable way to tell whether a positive skin test came from the vaccine or from actual TB infection. The BCG vaccine does not affect blood test results, which is why blood tests are the preferred option for anyone who has been vaccinated with BCG. This includes children, who in some countries receive BCG shortly after birth and can show skin test reactivity for the first two years afterward.
What a Positive Result Means
A positive TB screening result usually means your body has been infected with TB bacteria at some point. It does not mean you have active TB disease. The next step is a chest X-ray, which helps determine whether the infection is latent (inactive) or has developed into active disease.
Most people who test positive have latent TB infection. With latent TB, the bacteria are alive in your body but essentially dormant. You feel completely fine, have no symptoms, and cannot pass TB to other people. A diagnosis of latent TB infection is made when you have a positive screening test but your medical evaluation, including chest imaging, shows no signs of active disease.
Active TB disease is different. People with active TB typically feel sick, often with a persistent cough, fever, night sweats, or unexplained weight loss. They may be able to spread TB bacteria to others through the air. If active disease is suspected, providers will order additional tests beyond the chest X-ray, including sputum samples, without waiting for screening test results.
Why Latent TB Still Matters
Even though latent TB doesn’t make you sick or contagious, it carries a risk: the bacteria can reactivate later and develop into active disease. Treatment for latent TB significantly reduces that risk. This is the whole point of TB screening as a public health strategy. Finding and treating latent infections prevents future cases of active, contagious TB.
The risk of reactivation is especially high for people with weakened immune systems. HIV infection, organ transplants, severe kidney disease, diabetes, certain cancer treatments, corticosteroid use, and immunosuppressive therapies for conditions like rheumatoid arthritis or Crohn’s disease all raise the odds that a latent infection will progress. For these individuals, getting screened and treated for latent TB is particularly important.
Who Needs TB Screening
TB screening is typically targeted toward people with a higher chance of exposure or a higher risk of developing active disease if infected. You’re more likely to need screening if you:
- Were born in or travel frequently to countries where TB is common, particularly in parts of Asia, Africa, and Latin America
- Live or have lived in group settings such as homeless shelters, prisons, or jails
- Work in high-risk environments like hospitals, nursing homes, correctional facilities, or homeless shelters
- Have spent time with someone who has active TB disease
- Have a weakened immune system due to HIV, medication, or other health conditions
- Are a young child, since children under five have a higher risk of progressing to active disease once infected
Many employers in healthcare, education, and corrections require TB screening as part of onboarding. Some colleges and universities require it for enrollment. In these cases, either the skin test or the blood test is generally acceptable, though your workplace or school may specify which one they prefer.
What to Expect During the Process
If you’re getting a skin test, the injection itself takes only a few seconds and feels like a brief pinch. You’ll see a small raised bump on your forearm immediately afterward. Over the next two to three days, your body either reacts to the injected protein or it doesn’t. You must return to have the site read within that 48- to 72-hour window. If you miss the reading appointment, the test has to be repeated.
For a blood test, the experience is the same as any routine blood draw. You give one sample, and the lab handles the rest. There’s no return visit, which makes it more convenient for people with unpredictable schedules.
If your screening comes back negative, you’re done, at least until your next required screening. If it comes back positive, expect a chest X-ray as the next step. If the X-ray is clear and you have no symptoms, you’ll likely be diagnosed with latent TB infection and offered a course of treatment to prevent future activation. Treatment regimens for latent TB have gotten shorter in recent years, with some lasting as few as three to four months.