A few years back (probably more like 10) when the prisons really started regularly doing ppds, we ran into this issue. The best I can recall, we obtained the information from our State Dept of Health. Why? Well, we were trying to learn if it was nurse error when several inmates showed up positive when the year before they had been negative. Of course this caused quite a stir. What we learned was this....If you have a person that has not had a TB test for years, they may not react because of the body's immune system. In short, it has gone to sleep as far as TB exposure. To retest those people at a later date, awakens the immune response (memory cells)and may give a more accurate picture of a past exposure. Indeed we found that the only ones this had happen to were those in the 50-60 age group, who had never been tested, or had not been tested since they were young, and indeed had been exposed to a grandparent, parent, etc as a child. The site you were given by mollyj on the CDC website is excellent. I may add that I work in an Infectious Disease clinic with a 100% immune compromised clientele, and we do not use the 2-step, nor do we use anergy testing any longer (this rarily is done). Hope this helps you. You may want to check under Infectious Diseases with the CDC, since TB has greatly increased with the growing numbers of HIV/AIDS infected persons.