Infection with Tb. When someone is infected with tuberculosis (TB) germs, the body develops a protective response against them. The TB skin test is a test of the body's response to tb, not the germs themselves. One can have an active infection where they are coughing out TB germs and are a risk to spread tb. Or the body may have had the infection long ago and controlled it so there is no risk now of spread.
There. There are several different types of TB test, but the most common of which I think you are referring to is a skin prick test using a purified protein derivative (or ppd, for short). Frequently the situation to which you are referring to is called having a "positive ppd". Getting a PPD involves having a small injection applied to surface layer of the skin, usually the forearm. After 48-72 hours, a qualified professional has to measure the swelling or immune response at the skin in order to determine if it is a positive or negative test. Timing the reading of the PPD is very important, as is also the skill of the interpreter measuring the skin response. A test is only considered positive if it is measured (in millimeters) to be greater than 15. However, in some cases the test is considered positive if it is greater than 10 (e.g. In healthcare workers) and sometimes even if it is greater than 5 (e.g. In hiv-infected individuals). The interpretation of a PPD should therefore be ideally performed by a professional who has experience in diagnosing TB and has knowledge about your past medical history. Having a positive PPD generally means one of three things: a) you have been infected with TB at some time in the past, or b) you have been vaccinated with the bcg vaccination during childhood, or c) you have been exposed to other types of (generally benign) mycobacteria other than tb. There are additional blood tests available that can differentiate between the above.