What the tuberculosis test does

A tuberculosis test shows whether you have been infected with the bacteria that causes TB. It does not tell you whether you are sick right now or whether you will get sick later — only that the bacteria is or was in your body. Most people who test positive never develop active TB disease. The test is the first step a doctor takes when TB is suspected, or when you have been around someone with active TB.

There are two main types of TB tests: a skin test and a blood test. Both measure your immune system's response to TB bacteria. Your doctor will choose which one based on your age, health history, and why you are being tested.

Key Takeaways

  • The TB skin test (Mantoux test) involves an injection under the skin on your forearm, and you return 48 to 72 hours later to see if a bump formed.
  • The TB blood test (IGRA) measures immune cells in a blood sample and gives results in one to two weeks without a follow-up visit.
  • A positive test means TB bacteria is in your body, but a chest X-ray is needed to tell whether you have active TB disease or latent TB infection.
  • People with weakened immune systems, recent TB exposure, or symptoms like persistent cough should be tested.
  • If you test positive, your doctor will order imaging and possibly other tests to decide whether you need TB treatment.

The skin test (Mantoux test)

The Mantoux test is the oldest and most common TB test worldwide. A nurse injects a small amount of fluid called tuberculin just under the skin on your inner forearm. You feel a small prick, and a pale bump appears immediately — this is normal and not the test result.

You must return 48 to 72 hours later. The nurse measures the bump (called an induration) with a ruler. The size of the bump, measured in millimeters, tells the doctor whether you have been infected. A bump of 5 mm or larger is usually considered positive, though the threshold changes based on your risk factors and health history. Some people have no bump at all, which is a negative result.

The skin test is inexpensive and requires no blood draw, which makes it popular in clinics and schools. The main drawback is that you must return for a second visit, and some people forget or cannot get back in time.

The blood test (IGRA)

An interferon-gamma release assay (IGRA) is a blood test that measures how your immune cells respond to TB antigens — pieces of the TB bacteria. A technician draws blood into special tubes, sends it to a lab, and results come back in one to two weeks. You do not need a follow-up appointment.

The blood test is more convenient than the skin test because you get one visit and no return trip. It is also more specific, meaning fewer false positives in people vaccinated against TB (the BCG vaccine, common outside the United States). The main drawback is cost — blood tests are more expensive than skin tests — and a longer wait for results.

Both the skin test and blood test are considered reliable. Your doctor may choose one over the other based on what is available, your medical history, or whether you are likely to return for a follow-up visit.

What a positive test means

A positive TB test means TB bacteria has infected your body at some point. It does not mean you have active TB disease or that you are contagious. Most people with a positive test have latent TB infection — the bacteria is dormant in their lungs and they have no symptoms.

To find out whether you have active TB disease or latent TB infection, your doctor will order a chest X-ray. The X-ray shows whether there is damage or inflammation in your lungs. If the X-ray is normal and you have no symptoms, you almost certainly have latent TB. If the X-ray shows changes or you have symptoms like a persistent cough, fever, or night sweats, you may have active TB disease and will need treatment.

Your doctor may also ask about symptoms and risk factors. People with latent TB who have a high risk of developing active disease — such as those with HIV, those taking immunosuppressant drugs, or young children — are often treated with preventive medicine to stop the infection from becoming active.

What a negative test means

A negative test means your immune system did not react to TB bacteria, which usually means you have not been infected. However, a negative test does not rule out active TB disease in rare cases. If you have symptoms of TB (cough lasting more than three weeks, fever, night sweats, weight loss) and a negative test, your doctor may still order a chest X-ray or sputum test to be certain.

A negative test is also possible in people with weakened immune systems who cannot mount an immune response, even if they are infected. People with advanced HIV, severe malnutrition, or those taking certain medications may have a false negative. If you are in one of these groups and have been exposed to TB, your doctor may repeat the test or use other methods to check for TB.

Who should be tested

TB testing is recommended for people with symptoms of TB, those who have been in close contact with someone who has active TB, and those with risk factors for TB disease. Healthcare workers, people living with HIV, and people in congregate settings (prisons, homeless shelters, nursing homes) are often tested regularly.

People born in countries where TB is common, those who have lived or worked in high-risk settings, and those with medical conditions that weaken the immune system should discuss TB testing with their doctor. Your doctor 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. If your test is positive, your doctor will order a chest X-ray and ask about symptoms. Based on the X-ray and your symptoms, you will be diagnosed with either latent TB infection or active TB disease.

If you have latent TB, your doctor will discuss whether preventive treatment is right for you. If you have active TB disease, you will start a course of antibiotics that usually lasts four to six months. You will also be asked about close contacts — family members, coworkers, or others you spend time with — so they can be tested and treated if needed.

Frequently Asked Questions

Can I eat or drink before a TB test?

There are no food or drink restrictions before either the skin test or blood test. You can eat normally and take your regular medications. If you are having blood drawn, it is fine to have eaten beforehand.

What if I had the BCG vaccine — will it affect my test?

The BCG vaccine can cause a positive skin test even if you have never been infected with TB. If you were vaccinated and have a positive skin test, a blood test (IGRA) is more reliable because it is less affected by the vaccine. Tell your doctor if you received BCG so they can interpret your results correctly.

How long does it take to get TB test results?

The skin test requires a return visit 48 to 72 hours after the injection. The blood test takes one to two weeks for results. If you need results quickly, ask your doctor which test is available fastest in your area.

Can I shower or bathe after the skin test?

Yes, you can shower or bathe normally after the skin test. Just avoid scrubbing or scratching the injection site on your forearm. Keep the area clean and dry until your follow-up visit.

What if I test positive but have no symptoms?

A positive test with no symptoms usually means latent TB infection. Your doctor will order a chest X-ray to confirm. If the X-ray is normal, you have latent TB and may be offered preventive treatment to lower your risk of developing active disease later.