A TB test checks whether your body has been infected with the bacteria that cause tuberculosis. There are two main types: a skin test, where a small amount of protein is injected into your forearm, and a blood test, where a single blood draw is analyzed in a lab. Both work by detecting your immune system’s response to TB bacteria rather than looking for the bacteria directly. Most people encounter TB testing as a requirement for a new job, school enrollment, or healthcare work.
The Two Types of TB Tests
The skin test and the blood test accomplish the same goal through different methods. Which one you get often depends on your medical history, your healthcare provider’s preference, and practical considerations like whether you can return for a follow-up visit.
The skin test (also called the Mantoux test or TST) involves a tiny injection just under the skin of your inner forearm. A healthcare worker uses a small needle to place 0.1 ml of a purified protein from TB bacteria beneath the surface of your skin. It creates a small, pale bump that fades within minutes. You then need to return 48 to 72 hours later so a trained reader can examine the injection site and measure any firm, raised area (called an induration) that developed. Redness alone doesn’t count. Only the raised, firm bump is measured.
The blood test (called an IGRA, or interferon-gamma release assay) requires a single visit. A standard blood draw is sent to a lab, where your blood is mixed with synthetic proteins that mimic TB bacteria. If your immune system has encountered TB before, certain white blood cells will react by releasing a chemical signal called interferon-gamma. The lab measures the level of that signal to determine whether you’ve been infected. Results typically come back within a few days.
How the Skin Test Is Read
This is where TB testing gets more nuanced than most people expect. A positive skin test isn’t just “yes” or “no.” The size of the raised bump matters, and the threshold for a positive result depends on your personal risk factors.
- 5 millimeters or more is positive for people with HIV, organ transplant recipients, anyone on immunosuppressive medications, people who recently had close contact with someone with active TB, or anyone whose chest X-ray shows signs of past TB.
- 10 millimeters or more is positive for people born in countries where TB is common (including parts of Asia, Africa, and Latin America), people who live or work in nursing homes, homeless shelters, or correctional facilities, children under 5, people with diabetes, severe kidney disease, or certain cancers, and those with low body weight.
- 15 millimeters or more is positive for people with no known risk factors for TB.
This tiered system exists because a lower threshold catches more infections in people who face the greatest danger from TB, while a higher threshold reduces unnecessary follow-up testing in people at low risk.
Why the Blood Test Is Preferred After BCG Vaccination
Millions of people worldwide received the BCG vaccine as children. It’s a tuberculosis vaccine given routinely in many countries but not in the United States. If you were vaccinated with BCG, the skin test can produce a false positive, showing a reaction even though you don’t actually have a TB infection. There’s no reliable way to tell whether a positive skin test came from the vaccine or from real infection.
The blood test solves this problem. It uses proteins found specifically in TB bacteria but absent from the BCG vaccine strain. BCG vaccination does not cause a false positive on a blood test. If you were born in a country where BCG is standard or you know you received the vaccine, the blood test is the better option for you.
What a Positive Result Means
A positive TB test, whether skin or blood, tells you one thing: TB bacteria are in your body. It does not tell you whether you have latent TB infection or active TB disease, and the distinction between the two is critical.
With latent TB, the bacteria are present but dormant. You feel fine, have no symptoms, and cannot spread TB to anyone else. With active TB disease, the bacteria are multiplying and causing illness, typically in the lungs. Symptoms include a persistent cough, fever, night sweats, and unexplained weight loss. Active TB is contagious.
After a positive screening test, the standard next step is a chest X-ray. If the X-ray looks normal, you most likely have latent TB. If it shows abnormalities, or if you have symptoms, your provider will collect sputum samples (phlegm you cough up) for laboratory analysis. A bacterial culture of the sputum is the gold standard for confirming active TB disease.
Who Should Get Tested
TB testing isn’t recommended for the general population. The U.S. Preventive Services Task Force recommends screening for adults at increased risk, which includes several specific groups. People born in or who previously lived in countries with high TB rates are at the top of the list. So are people who live or work in congregate settings like homeless shelters, correctional facilities, and nursing homes.
Healthcare workers and anyone who has had close contact with a person diagnosed with active TB should also be tested. People with weakened immune systems, whether from HIV, organ transplantation, chemotherapy, or medications that suppress immune function, face a higher risk of latent TB progressing to active disease, making screening especially important. People with silicosis, a lung disease caused by inhaling silica dust, are also in this higher-risk category.
Many people get tested not because of personal risk factors but because an employer, school, or immigration process requires it. If that’s your situation, either test type will work, though your provider may recommend the blood test if you’ve had a BCG vaccine or if returning for a 48-to-72-hour reading isn’t practical.
How Long It Takes for a Test to Work
Neither test can detect a brand-new infection. After exposure to TB bacteria, your immune system needs six to eight weeks to develop the specific response that these tests measure. If you were recently exposed to someone with active TB, a negative test taken too early could be misleading. In that scenario, you may need to be retested after the window period has passed.
For the skin test, you’ll know your result at the reading appointment, 48 to 72 hours after the injection. Missing that window means the test can’t be accurately read and you’ll need to start over. For the blood test, results are typically available within one to three days, depending on the lab.