An Overview of the TB Test
There are two types of TB (
tuberculosis) tests. One is a TB skin test (also known as a PPD test) and the other is a TB blood test (QuantiFERON
®-TB Gold).
What Is the TB Skin Test?
A tuberculosis skin test is a TB test that is often used to detect
latent tuberculosis. You can get a skin test at the health department or at your doctor's office. A healthcare worker will inject a small amount of testing fluid (called tuberculin or PPD) just under the skin on the underside of the forearm. After 2 or 3 days, you must return to have your skin test read by the healthcare worker. You may have a swelling where the tuberculin was injected. The healthcare worker will measure this swelling and tell you if your reaction to the tuberculosis skin test is positive or negative. A positive reaction usually means that you have been infected by someone with
active tuberculosis.
If you have recently spent time with and been exposed to someone with active TB, your TB skin test reaction may not be positive yet. You may need a second skin test 8 to 10 weeks after the last time you spent time with the person. This is because it can take several weeks after infection for your immune system to react to the TB skin test. If your reaction to the second test is negative, you probably do not have latent TB infection.
(Click TB Skin Test for more information about this TB test.)
QuantiFERON
®-TB Gold (QFT) is a blood test used to find out if you are infected with TB bacteria (
Mycobacterium tuberculosis). The QFT measures the response to TB proteins when they are mixed with a small amount of blood. Currently, few health departments offer the QFT. If your health department does offer the QFT, only one visit is required. Your blood will be drawn at that visit for the test.
You should get a TB test if:
- You have spent time with a person known to have active tuberculosis or suspected to have active tuberculosis
- You have HIV infection or another condition that puts you at high risk for active tuberculosis
- You think you might have active tuberculosis
- You are from a country where active tuberculosis is very common (most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia)
- You live somewhere in the United States where active tuberculosis is more common (such as a homeless shelter, migrant farm camp, prison or jail, or some nursing homes)
- You inject illegal drugs.
If you have a positive reaction to the TB skin test or the QFT, your doctor or nurse may do other tests to see if you have active tuberculosis. These tests usually include a chest x-ray and a test of the phlegm you cough up. Because the TB bacteria may be found somewhere other than your lungs, your doctor or nurse may check your blood or urine, or do other tests. If you have active tuberculosis, you will need to take medicine to cure the disease.
If you have latent TB infection (a positive TB skin test reaction or positive QFT), and you are in a high-risk group
(see Causes of Tuberculosis), you need to take
tuberculosis medications to keep from developing active tuberculosis. This is called latent
tuberculosis treatment. There are several treatment options. You and your healthcare provider must decide which treatment is best for you.
