TB screening is a testing strategy used to detect tuberculosis infection in people who may not have any symptoms. The goal is to find the infection early, particularly in its latent (inactive) stage, before it can progress to active disease or spread to others. Two main tests are used: a skin test and a blood test. Both detect your immune system’s response to TB bacteria rather than the bacteria themselves.
Latent TB vs. Active TB Disease
Understanding the difference between these two stages is key to understanding why screening matters. With latent TB infection, TB bacteria live in your body in an inactive state. You feel completely fine, have no symptoms, and cannot pass TB to anyone else. You could carry latent TB for years without knowing it. The risk is that latent TB can “wake up” and become active disease, especially if your immune system weakens.
Active TB disease is different. You typically feel sick, often with a persistent cough, fever, night sweats, and unexplained weight loss. Active TB can spread to others through the air when you cough or sneeze. TB screening is designed primarily to catch the latent stage, so treatment can prevent it from ever reaching this point.
The Two Types of TB Tests
TB Skin Test (TST)
The TB skin test, also called the Mantoux test, involves a small injection just under the skin on the inside of your forearm. A healthcare provider injects a tiny amount of a protein derived from TB bacteria. If your immune system has encountered TB before, the injection site will swell into a firm, raised bump over the next two to three days.
You must return to a healthcare provider between 48 and 72 hours after the injection to have the result read. This is non-negotiable: a trained provider needs to measure the size of any raised area at the injection site. If you don’t return within that 72-hour window, the test is invalid and you’ll need to start over. The size of the bump, measured in millimeters, determines whether the result counts as positive. Different thresholds (5 mm, 10 mm, or 15 mm) apply depending on your individual risk factors.
TB Blood Test (IGRA)
The alternative is a blood test called an interferon-gamma release assay, or IGRA. A standard blood draw is taken, and the sample is mixed in a lab with synthetic proteins that mimic TB bacteria. If your white blood cells have been “trained” by a previous TB encounter, they release a chemical signal called interferon-gamma. The lab measures how much of this signal appears. Two FDA-approved versions are available in the U.S.: QuantiFERON-TB Gold Plus and T-SPOT.TB.
The blood test has a practical advantage: it requires only one visit. There’s no return appointment to read results. It also tends to be more accurate for people who received the BCG vaccine, a TB vaccine commonly given in many countries outside the U.S. The BCG vaccine can cause a false positive on the skin test because it primes the immune system in a similar way, but the blood test uses proteins that are specific to actual TB bacteria and absent from BCG. This makes the blood test a better choice for anyone vaccinated against TB as a child.
Who Should Be Screened
TB screening is targeted, not universal. It focuses on people who have a higher chance of being exposed to TB or a higher chance of getting seriously sick if infected. You’re more likely to have been exposed if you were born in or frequently travel to countries where TB is common (parts of Asia, Africa, and Latin America), live or have lived in group settings like homeless shelters or correctional facilities, or work in hospitals, nursing homes, or other healthcare environments.
Certain health conditions also raise the stakes. HIV infection, diabetes, severe kidney disease, organ transplants, and treatments that suppress the immune system (such as medications for rheumatoid arthritis or Crohn’s disease) all increase the risk that latent TB will progress to active disease. Injection drug use, low body weight, and cancers of the head and neck are also risk factors. Children under five are especially vulnerable because their immune systems are still developing.
Screening in Children
For young children, TB testing comes with some additional considerations. Blood tests are less reliable in kids under five because their developing immune systems produce weaker and less consistent responses. Research shows that the sensitivity of blood tests in this age group is particularly low, meaning the test may miss infections that are actually there. For young children, the skin test is generally considered safer as a first-line option. In children who received the BCG vaccine, using both the skin test and blood test together can help sort out true infections from vaccine-related reactions, reducing unnecessary treatment.
What Happens After a Positive Result
A positive skin test or blood test means your immune system has encountered TB bacteria at some point. It does not tell you whether you have latent infection or active disease. That distinction requires further evaluation.
The next step is a chest X-ray. If the X-ray looks normal and you have no symptoms (no cough, fever, night sweats, or weight loss), the diagnosis is latent TB infection. You’ll likely be offered treatment to clear the inactive bacteria and prevent future disease. If the X-ray shows abnormalities, or if you do have symptoms, additional testing follows. This can include collecting sputum (mucus coughed up from the lungs) so a lab can look for and grow the bacteria directly, confirming active disease and determining which medications will work against it.
Once you’ve tested positive, repeat screening with a skin test or blood test isn’t useful going forward, since it will always come back positive. Instead, future monitoring relies on chest X-rays and symptom checks as needed.
How Often Screening Is Needed
There’s no single rule for how often to get screened. The U.S. Preventive Services Task Force notes that screening frequency should match your ongoing risk. If you have a one-time risk factor, like past travel to a high-prevalence country but no continuing exposure, a single screening may be sufficient. If you work in healthcare or another setting with ongoing TB exposure, annual screening is typical. Many employers in healthcare, corrections, and homeless services require TB screening as a condition of employment, often at hire and then periodically afterward.
For healthcare workers specifically, baseline screening at the start of employment involves an initial test. For those at low risk, a second test is done to confirm the result before any positive finding is acted on. This two-step approach reduces the chance of a false positive driving unnecessary follow-up.
What the Experience Is Like
If you’re getting a skin test, expect a brief pinch on the inside of your forearm. The injection creates a small, pale bump about the size of a pencil eraser. It fades quickly. Over the next two to three days, you may notice redness or a raised area forming at the site if your body reacts. Don’t cover, scratch, or put lotion on the area, as this can interfere with the reading. At your follow-up visit, the provider will press the site gently to feel for any firm swelling and measure it.
If you’re getting a blood test, it’s a standard blood draw from your arm. Results typically come back within a few days. There’s no second appointment needed, which makes this option more convenient, especially if returning for a reading would be difficult.
Neither test can give you TB. The skin test uses an inactive protein fragment, not live bacteria, and the blood test analyzes a sample in a lab. Both tests detect your immune system’s memory of past exposure. After exposure to TB bacteria, that immune memory typically develops within six to eight weeks, so testing too soon after a known exposure may produce a false negative.