A tuberculosis (TB) test can be effective in determining whether a person has been infected with the tuberculosis bacteria. There are two types of this test: the skin test and the blood test. During the TB skin test, a testing fluid is injected into the forearm, and a healthcare provider will measure the swelling after 72 hours and determine the results. A TB blood test measures the response to TB proteins when they are mixed with a small amount of your blood.
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.)