TB Skin Test
This is the most common way doctors diagnose tuberculosis. A tiny amount of fluid called tuberculin gets injected just below the skin in your forearm. It contains some inactive TB bacteria. 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 if 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 necessarily mean you have active tuberculosis disease.
If you don’t have a reaction, your test is "negative" and 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 have a skin test again because it can trigger a painful reaction where you get the shot.
Sometimes a doctor will repeat a TB skin test. The test might falsely show you don't have TB, especially if you've been exposed to TB in the past. A false positive test may also happen if you've been vaccinated with the TB bacillus Calmette-Guerin (BCG) vaccine.
If your first test was negative, a second test can be done a week or two later on your other arm. If the second one is positive, you'll need more tests.
TB Blood Tests
Two types can identify a TB infection. 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 be given other tests to see if your tuberculosis is active.
Latent TB Infection vs. TB Disease
If you have tuberculosis, you might have a latent TB infection or active TB disease.
With a latent 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 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 may include:
People with TB can spread it to other people. They'll usually have positive TB skin or blood tests. They may also have abnormal chest X-rays or sputum tests.
TB disease is serious and needs treatment.
Testing for Drug Resistance
Your doctor will test early on to make sure your TB isn't resistant to the drugs you're taking for it. Sometimes, if you miss a dose or take your medicine incorrectly, your TB is harder to treat the second time. The drugs aren't able to fight the TB.
TB that has become resistant to drugs is harder to treat, and you may need more medication for a longer time.