A TB test checks whether the bacteria that cause tuberculosis are living in your body. There are two main types: a skin test that requires two visits to a clinic, and a blood test that needs only one. Both detect your immune system’s response to TB bacteria rather than the bacteria themselves, and neither can tell you whether an infection is dormant or actively making you sick.
Why You Might Need One
TB testing is common for new jobs, school enrollment, and healthcare positions. Beyond those routine requirements, you’re more likely to need a test if you’ve been in close contact with someone who has active TB, or if your daily life puts you at higher exposure risk. That includes working or living in jails, nursing homes, or homeless shelters, traveling frequently to countries where TB is widespread, or injecting drugs with shared needles.
Certain health conditions also raise your risk enough to warrant testing. People with weakened immune systems, diabetes, or HIV are more vulnerable, as are those receiving treatments for rheumatoid arthritis or Crohn’s disease, or anyone preparing for an organ transplant. If you have symptoms of active TB, a cough lasting more than three weeks, coughing up blood, chest pain, night sweats, unexplained weight loss, or persistent fever, testing becomes urgent.
The Skin Test (Mantoux Test)
The skin test is the older and more widely recognized method. A healthcare worker injects a tiny amount of purified TB protein just under the skin of your inner forearm, a few inches below the elbow. You’ll see a small, pale bump form at the injection site. That bump is normal and fades quickly.
The key part comes later. You must return to the clinic between 48 and 72 hours after the injection so a trained reader can check your arm. They’re feeling for a specific reaction: a firm, raised area called induration. Redness alone doesn’t count. The reader measures the width of the raised area in millimeters and records that number. If you miss the 72-hour window, the test can’t be read and you’ll need to start over with a new injection. There’s no health risk from repeating the test.
What counts as “positive” depends on your personal risk level. A smaller area of induration can be significant for someone with HIV or a suppressed immune system, while a larger threshold applies to people with no known risk factors. Your provider interprets the measurement in the context of your health history.
One Important Limitation
If you’ve ever received the BCG vaccine, which is given routinely to children in many countries outside the United States, your skin test may come back positive even if you don’t have TB. There’s no reliable way to tell whether a positive reaction is from the vaccine or from a real infection. The more BCG doses you’ve received, and the more recently you got them, the more likely this false positive becomes. Tuberculin reactivity from the vaccine tends to fade over time, but repeated skin testing can actually keep it going.
The Blood Test (IGRA)
TB blood tests, called interferon-gamma release assays, work differently. A single blood draw is all that’s needed. In the lab, your blood sample is mixed with synthetic proteins that mimic TB bacteria. If your white blood cells have encountered TB before, they release a chemical signal in response. The lab measures that signal to determine your result.
Two versions are approved in the U.S.: QuantiFERON-TB Gold Plus and T-SPOT.TB. Both accomplish the same goal through slightly different lab techniques. QuantiFERON measures the total amount of immune signal released, while T-SPOT counts the individual cells producing it.
Blood tests have some practical advantages. They require only one visit, they aren’t affected by BCG vaccination, and they remove the human error involved in placing and reading a skin test. The CDC recommends blood tests as the preferred option for anyone who has received the BCG vaccine, including children. Blood tests also tend to be more sensitive at detecting TB infection. In comparative studies, blood tests identified infections at roughly twice the rate of skin tests in high-risk populations.
What Neither Test Can Tell You
Both the skin test and the blood test detect TB infection, meaning your immune system has encountered the bacteria at some point. What they cannot do is distinguish between latent TB infection and active TB disease. This is a critical difference. Latent TB means the bacteria are in your body but dormant: you feel fine, you’re not contagious, and you may never get sick. Active TB means the bacteria are multiplying, causing symptoms, and potentially spreading to others.
Separating latent from active TB requires different tools entirely. Lab techniques that look for the bacteria directly, through sputum samples examined under a microscope and grown in culture, are the standard for making that distinction.
What Happens After a Positive Result
A positive screening test is not a TB diagnosis. It means your body has been exposed to TB bacteria and your immune system reacted. The next step is a chest X-ray to look for signs of active disease in the lungs.
If your chest X-ray looks normal and you have no symptoms, you’ll typically be evaluated for treatment of latent TB infection. Treatment for latent TB is preventive: it aims to kill dormant bacteria before they ever become active. If your X-ray shows anything abnormal, or if you’re experiencing symptoms, you’ll be asked to provide sputum samples for lab analysis and possibly undergo additional testing to determine whether you have active disease.
Choosing Between the Two Tests
For most people, either test is acceptable. The choice often comes down to logistics and your medical history. If you’ve had the BCG vaccine, a blood test avoids the ambiguity of a potentially false-positive skin test. If you’d rather not make two clinic visits, the blood test is more convenient. Some workplaces or schools may specify which test they require, so it’s worth checking before your appointment.
Children, people with compromised immune systems, and those recently exposed to someone with active TB may benefit from a blood test’s higher sensitivity. Your provider can help you decide based on your specific situation and the reason for testing.