The two main ways to test for tuberculosis

Doctors use two different tests to learn about you have tuberculosis: a skin test (also called the Mantoux test or tuberculin skin test) and a blood test (called an interferon-gamma release assay, or IGRA). Both detect whether your immune system has been exposed to the bacteria that causes TB. The skin test has been used for over a century and remains common. Blood tests are newer and are becoming more widely available.

Neither test tells you whether you have active TB disease or latent TB infection—the dormant form where bacteria live in your body but you have no symptoms and cannot spread the disease. A positive result on either test means you need a follow-up chest X-ray or other imaging to determine which form you have. That distinction matters because only active TB requires treatment to prevent spread.

Key Takeaways

  • The skin test involves injecting a small amount of TB protein under your skin and returning 48 to 72 hours later to see if a bump forms.
  • Blood tests measure how your immune cells react to TB antigens in a lab and give results within a few days, without a second visit.
  • A positive test does not mean you have active TB—you will need a chest X-ray to find out whether the infection is active or dormant.
  • Skin tests can give false results if you have received certain vaccines or have a weakened immune system, which is why blood tests are sometimes preferred.
  • If you test positive, your doctor will ask about symptoms like cough, fever, and weight loss to help decide what happens next.

How the tuberculin skin test works

A healthcare provider injects a small amount of purified TB protein (called tuberculin) just under the skin, usually on your forearm. The injection itself is painless—you may feel a slight prick. The provider marks the spot or writes down its location so you know where to return.

You must come back 48 to 72 hours later—exactly two to three days—for the reading. At that visit, the provider measures any bump (called an induration) that formed at the injection site. A bump larger than a certain size means a positive result, though the size threshold depends on your risk factors. Someone with HIV or recent TB exposure needs a smaller bump to be considered positive than someone with no known risk. If there is no bump or only redness, the test is negative.

The main drawback is that you need two visits. If you miss the second appointment, the test is invalid and you have to start over. The skin test can also give false positives if you received the BCG vaccine (common outside the United States) or false negatives if your immune system is very weak.

How blood tests for TB work

A phlebotomist draws a small sample of blood, usually from your arm. The blood goes to a lab where technicians mix it with TB antigens—pieces of the TB bacteria—and measure how strongly your immune cells respond. The lab looks for interferon-gamma, a chemical that immune cells release when they recognize TB.

Results typically come back within one to three days. You do not need a second visit. Blood tests are more specific than skin tests, meaning they are less likely to give a false positive from BCG vaccination or other causes. They also work better in people with weakened immune systems, though they can still be falsely negative in severe immunosuppression.

The drawback is cost—blood tests are more expensive than skin tests—and not all clinics or urgent care centers have the equipment to run them. Availability varies by region and by healthcare setting.

What a positive test result means

A positive skin test or blood test means your immune system has encountered TB bacteria at some point. It does not tell you when, whether you are contagious, or whether you will ever develop symptoms. About 5 to 10 percent of people with latent TB infection eventually develop active disease, usually within the first two years after infection.

Your doctor will ask about symptoms: a cough lasting more than three weeks, chest pain, fever, night sweats, or unexplained weight loss. If you have none of these and your chest X-ray is normal, you have latent TB infection. If your X-ray shows shadows or infiltrates in your lungs, or if you have symptoms, you have active TB disease and need treatment to prevent spread to others.

What a negative test result means

A negative result means your immune system did not react to TB antigens, which usually means you have never been infected. However, a negative result does not completely rule out TB in certain situations. Someone who was recently exposed to TB may not yet have developed an immune response—the window can be up to eight weeks. A person with a very weak immune system (such as advanced HIV) may not mount a detectable response even if infected.

If you have symptoms of TB and a negative test, your doctor may still order a chest X-ray or sputum test (coughing up a sample for the lab to examine). These tests look for the bacteria directly rather than measuring immune response.

Who should be tested for TB

Testing is recommended for people with symptoms of TB, those who have spent time with someone who has active TB, healthcare workers, people living with HIV, and people in congregate settings like shelters or prisons. Testing may also be done before starting certain medications that weaken the immune system, or before major surgery.

Routine screening of the general population is not standard practice in the United States because TB is uncommon. Your doctor or local health department can tell you whether testing makes sense for your situation.

What happens after testing

If your test is negative and you have no symptoms, no further action is needed unless you were recently exposed—in that case, your doctor may recommend a repeat test in eight weeks to allow time for an immune response to develop.

If your test is positive, you will have a chest X-ray. If the X-ray is normal and you have no symptoms, you have latent TB infection. Your doctor will discuss whether preventive treatment is right for you. Preventive treatment uses one or more TB drugs for several months to reduce the risk that latent infection becomes active disease. If your X-ray shows signs of active disease or you have symptoms, you will need a sputum test to confirm TB and start treatment with multiple drugs over several months.

Frequently Asked Questions

Can I shower or bathe after a TB skin test?

Yes. Water will not affect the test. You can bathe, swim, or wash the area normally. Do not scratch or rub the injection site, as this can affect the bump that forms and make the reading inaccurate.

What if I cannot return for the skin test reading within 72 hours?

The reading must happen between 48 and 72 hours for accuracy. If you miss this window, the test is invalid and you will need to start over with a new injection. Plan ahead to make sure you can return on time, or ask your doctor about a blood test instead, which does not require a second visit.

Does a positive TB test mean I have active tuberculosis?

No. A positive test means you have been exposed to TB bacteria, but it does not tell you whether the infection is active or dormant. Your doctor will use your symptoms, a chest X-ray, and possibly a sputum test to determine whether you have active TB disease or latent TB infection.

Can I get TB from the test itself?

No. The skin test uses only a protein from the TB bacteria, not the live bacteria. The blood test uses antigens in a test tube, not live bacteria. Neither test can cause TB infection.

Why would my doctor choose a blood test instead of a skin test?

Blood tests are preferred if you have received the BCG vaccine, have a weakened immune system, or cannot return for a second appointment. They also give results faster and do not require you to remember to come back in two to three days.