TB (Tuberculosis) Tests

Medically Reviewed by Paul Boyce, MD on November 04, 2023
4 min read

Tuberculosis is not always easy to diagnose. Tuberculosis usually causes infection in the lungs, but not always. Tuberculosis can cause a latent (asymptomatic or quiet) infection or active tuberculosis which will require treatment. Your doctor will perform a physical exam and a skin or blood test to see if you may have tuberculosis. Additional tests are ordered depending on the results.

Many people who have TB don’t have any symptoms. They have what doctors call latent TB. The TB skin test, also known as the Mantoux tuberculin skin test, is the most common way doctors diagnose tuberculosis. They’ll inject a tiny amount of fluid called tuberculin just below the skin in your forearm. It contains some inactive TB protein. You should feel a small prick from the needle. You’ll go back to your doctor 2 or 3 days later, and a health care worker will see whether you’ve had a reaction.

If you have a raised, hard bump or there's swelling on your arm, you have a positive test. That means TB germs are in your body. But it doesn't always mean you have active tuberculosis disease.

Your doctor may do more tests to be sure. These could include blood tests or an X-ray of your chest. They may also want to test your sputum -- that’s the gunk you cough up. Urine or tissue samples can also tell your doctor if the TB germs have begun to spread, or become “active.”

If you don’t have a reaction (or if you have a very small one), your test is negative. You don't have TB germs in your body. But if you were infected recently, your immune system may not react to the skin test yet. In that case, your doctor may want you to have another TB skin test in 8 to 10 weeks.

If you've had a positive TB skin test in the past, you'll be expected to always test positive in the future. Once you have had a positive skin test, let your doctor know. They are usually not repeated because the swelling can be worse each time.

Sometimes a doctor will repeat a TB skin test. The test might show you don't have TB when you do, especially if you were exposed a long time ago and your immune response to it is weak. Or you could get a false positive result if you've been vaccinated with the TB bacillus Calmette-Guerin (BCG) vaccine.

If your first test was negative, you can get a second test a week or two later on your other arm. If the second one is positive, you'll need more tests.

Blood tests called interferon gamma release assays (IGRAs) measure your response to TB antigens, things that cause your immune system to make antibodies. Two tests have been approved by the FDA. You may have them instead of, or in addition to, a TB skin test.

Once you've had your blood test, you don't need another visit. They can help if you've had a negative TB skin test or if you've had the BCG vaccine.

If your blood test is positive, it means you've been infected with TB germs. You'll get other tests to see if your tuberculosis is active.

If you have a positive skin or blood TB test, your doctor may give you a chest X-ray. They’ll look for spots on your lungs or any changes caused by TB.

You may also take something called a sputum smear or culture test. Your doctor will take a sample of the mucus that comes up when you cough and test it for TB bacteria.

If you have tuberculosis, you might have a latent TB infection or active TB disease.

With a latent TB infection, you have the TB bacteria, but you don't feel sick and you have no symptoms. You can't spread TB to anyone else. The only sign that you have a TB infection is a positive TB skin test or blood test.

With latent infection, your chest X-ray will not show active TB and your sputum cultures will be negative for TB.

About 5% to 10% of people who have a latent TB infection will eventually get active TB disease. Your chance of it depends on your medical history.

In some cases, the TB bacteria overcome your body's immune system and multiply. This becomes TB disease. You'll have symptoms that can include:

If you have active TB, you can spread it to other people. Active TB is diagnosed by the presence of symptoms, TB tests, sputum cultures and imaging.. Work with your doctor if you get these test results. TB disease is serious and needs treatment.

The doctor will test early on to make sure your TB responds to the drugs you're taking for it. If it doesn’t, you could have what’s called drug-resistant TB. Sometimes, if you miss multiple doses or don't take your medicines like you're supposed to, your TB can develop resistance to some of the medications used to treat it.The drugs may not be able to fight the disease.

Drug-resistant TB is harder to treat, and you may need more medication for a longer time.