TB Skin Test
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.
TB Skin Testing Results
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.
If you don’t have a reaction, your test is negative. You don't have TB germs in your body.
If you've had a positive TB skin test in the past, you'll probably have a positive test again in the future. So there's no reason to get a skin test again.
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.
TB Blood 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.
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.
Other TB Tests
Latent TB Infection vs. Active TB Disease
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 a latent infection, you'll have a negative chest X-ray and a negative sputum test.
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. You probably have it if your chest X-rays or sputum tests show signs of TB. Work with your doctor if you get these test results. TB disease is serious and needs treatment.
Testing for TB Drug Resistance
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 a dose or don’t take your medicine like you’re supposed to, your TB will be harder to treat the second time. 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.