The most common test for tuberculosis is a PPD test, short for purified protein derivative test. This skin test helps our doctors detect if you have come into contact with the bacteria that causes TB. This test requires you to visit our location twice: Once for the test to be administered and a second time for the test to be read. When you visit any of our clinics, our staff will first talk to you about your medical history. If you were born outside of the United States, you might have received a vaccine for tuberculosis. This vaccine is not standard in the United States and may cause a false positive for the test. There are also some fields of work that requires this vaccine, so our staff will also ask you a few questions about what you do for a living. As long as you have not been given a vaccine, you will be given the test. A needle will be placed just under your skin, and the PPD will be injected to make a small welt or bump. This bump will go away after a few hours. After 48 hours, you must return to our clinic. You must return within 72 hours, or the test will not be valid. Our providers will "read" the test by measuring the reaction of the PPD on the skin. A negative reading is indicated when the skin shows no response, meaning an area that is not swollen or is only swollen by a tiny amount.