What a tuberculosis test does

A tuberculosis test detects whether you have been exposed to the bacteria that causes TB, or whether you have active TB disease. The test does not tell you whether you will get sick — only whether the bacteria is present in your body or whether you have the active infection. There are two main types: one checks for exposure (latent TB), and one checks for active disease.

Your doctor orders a TB test when you have symptoms like a persistent cough, chest pain, or fever; when you have been around someone with active TB; or as part of a routine screening for certain jobs or living situations. The test itself is quick and painless, though getting results takes a few days to a few weeks depending on which type you receive.

Key Takeaways

  • The tuberculin skin test (Mantoux test) involves an injection under the skin and requires you to return 48 to 72 hours later for a nurse to measure the reaction.
  • Blood tests for TB (IGRA tests) can be done in one visit and give results within one to two weeks, making them faster than skin tests for some people.
  • A positive test result means you have been exposed to TB bacteria, but does not automatically mean you have active disease or are contagious.
  • If your test is positive, your doctor will order a chest X-ray and possibly sputum tests to determine whether you have latent or active TB.
  • TB tests are often required for healthcare workers, teachers, and people living in congregate settings like shelters or correctional facilities.

The tuberculin skin test (Mantoux test)

The tuberculin skin test, also called the Mantoux test, is the most common TB screening method. A nurse injects a small amount of purified protein derivative (PPD) just under the skin on your forearm. The injection itself feels like a small pinch and takes less than a minute. You will see a small raised bump appear at the injection site, but this goes away within hours.

You must return to the clinic or doctor's office 48 to 72 hours later — this timing is important and cannot be rushed. A nurse will examine your arm and measure the size of any raised bump or hardening at the injection site. The measurement in millimeters determines whether your result is positive, negative, or uncertain. The size that counts as positive varies depending 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.

The skin test is inexpensive and widely available, but it requires two separate visits. Some people have a false positive if they received a BCG vaccine (common outside the United States) or if they have had TB in the past. Others have a false negative if their immune system is very weak or if they were tested too soon after exposure.

Blood tests for TB (IGRA tests)

Interferon-gamma release assays (IGRA) are blood tests that detect TB exposure by measuring how your immune cells respond to TB antigens. Common IGRA tests include the QuantiFERON-TB Gold and T-SPOT.TB. A nurse draws blood in a single visit, and you do not need to return for a second appointment.

IGRA results typically come back within one to two weeks. These tests are more specific than the skin test — they are less likely to show a false positive from a BCG vaccine — and they work better in people with weakened immune systems. However, they are more expensive than the skin test and not all clinics offer them. Some people prefer them because there is no waiting period or second visit required.

Like the skin test, a positive IGRA result means you have been exposed to TB bacteria, but it does not tell you whether you have active disease. Your doctor will still order follow-up tests if your result is positive.

What happens after a positive test

A positive TB test — whether skin or blood — means your body has been exposed to TB bacteria. It does not mean you have active TB disease or that you are contagious. About 5 to 10 percent of people with latent TB infection will develop active disease at some point in their lives, usually within the first two years after infection.

Your doctor will order a chest X-ray to look for signs of active TB in your lungs. If the X-ray is normal, you likely have latent TB infection. If the X-ray shows abnormalities, your doctor may order sputum tests — you cough into a cup and the sample is checked for TB bacteria under a microscope or through culture. Sputum tests take longer (culture can take two to eight weeks) but they confirm whether you have active disease and whether the bacteria is drug-resistant.

If you have latent TB, your doctor may recommend preventive treatment with antibiotics to reduce your risk of developing active disease later. If you have active TB, you will need a longer course of antibiotics, usually four drugs taken for two months followed by two drugs for four more months.

Who needs a TB test

TB testing is required or strongly recommended for certain groups. Healthcare workers, including doctors, nurses, and nursing home staff, must be tested before starting work and then periodically during employment. Teachers and school staff are often required to test. People living in congregate settings — shelters, correctional facilities, long-term care facilities — are tested regularly because TB spreads more easily in close quarters.

You may also need a test if you have been in close contact with someone who has active TB, if you have symptoms like a persistent cough lasting more than three weeks, or if you have a medical condition that weakens your immune system (HIV, diabetes, kidney disease). Some employers and housing programs require TB tests as part of their screening process.

If you are immigrating to the United States, you will need a TB test as part of your medical examination. Some countries require TB testing before you can enter or work there.

Cost and where to get tested

The cost of a TB test varies. The skin test typically costs $10 to $50 if you pay out of pocket, while blood tests range from $50 to $150. If you have health insurance, your plan usually covers TB testing with no out-of-pocket cost, especially if your doctor orders it for a medical reason. If you do not have insurance, many public health departments and community health centers offer TB testing for free or on a sliding fee scale based on income.

You can get a TB test at your primary care doctor's office, an urgent care clinic, a hospital, or a public health department. If you need a test and do not have a doctor, call your local health department — they can tell you where to go. Some workplaces and schools have on-site testing days. If you are being tested because of a work requirement, your employer may cover the cost and arrange the testing for you.

False positives and false negatives

A false positive means the test shows you have TB exposure when you actually do not. This happens most often with the skin test in people who received a BCG vaccine, which is standard in many countries outside the United States. It can also happen if you have a non-TB mycobacterial infection. Blood tests are more specific and less likely to give false positives from BCG.

A false negative means the test shows no TB exposure when you actually have been exposed. This is more common if you were tested too soon after exposure (before your immune system had time to react), if you have a very weak immune system, or if you have active TB disease but your immune response is not strong enough to show up on the test. People with HIV, especially those with very low CD4 counts, are more likely to have false negative results.

If your test result does not match your symptoms or your exposure history, your doctor may order a second test or a chest X-ray to clarify. Repeat testing is normal and does not mean something is wrong.

Frequently Asked Questions

Can I get a TB test if I have no symptoms?

Yes. TB testing is often done as a screening tool even when you have no symptoms — for work, school, immigration, or because you were exposed to someone with active TB. Many people with latent TB infection have no symptoms at all, which is why screening is important for certain groups.

How long does it take to get results?

Skin test results take 48 to 72 hours because you must return for the measurement. Blood test results typically come back within one to two weeks. If your test is positive and your doctor orders a chest X-ray or sputum culture, those results add another few days to a few weeks depending on the test.

What if my skin test is positive but my blood test is negative?

This can happen and does not necessarily mean one test is wrong. Your doctor will look at your symptoms, your exposure history, and your chest X-ray to decide what it means. Sometimes a repeat test or additional testing clarifies the result. Discordant results (one positive, one negative) are not uncommon and do not automatically mean you have active TB.

Can I work or go to school while waiting for TB test results?

This depends on your workplace or school policy and whether you have symptoms. If you have no symptoms and are waiting for results of a routine screening, you can usually continue normal activities. If you have symptoms of active TB (persistent cough, fever, chest pain), you should not go to work or school until active disease has been ruled out, because active TB is contagious.

Do I need a TB test every year?

This depends on your job and your risk. Healthcare workers and people in congregate settings are often tested annually or more frequently. If you had a negative test and have no ongoing exposure risk, you may not need another test. Your doctor or employer can tell you whether repeat testing is needed in your situation.