general population of the country on a yearly basis to detect this transmission would have a positive predictive value of less than 10 percent (see Table B-1).

Although skin testing with tuberculin does not induce enough immunologic challenge to induce a positive reaction on subsequent tests, waned delayed hypersensitivity reactions from remote infections with M. tuberculosis and cross-reactions to antigens from other mycobacteria can be boosted or enhanced.⁽²⁴⁾ Such boosting generally occurs within 1 week and may persist for a year or more. Boosted reactions are particularly common in people who have received BCG vaccination,⁽²⁰⁾ people with environmental exposure to other mycobacteria, and people from countries with a high prevalence of tuberculosis. False conversions due to boosting are particularly common in these populations when the second skin test is placed in a serial skin testing program.⁽²⁵⁾ In order to detect this phenomenon, it is recommended that serial skin testing programs use an initial two-step test in which a second skin test is placed approximately 1-week after the first if there is no reaction to the first and that significant reactions to the second test are considered boosted reactions rather than conversions. There is a suggestion that boosted reactions may continue to occur subsequent to the second test in persons who have been vaccinated with BCG and persons from high-prevalence countries, but no practical method of detecting these late boosters is available.

In considering serial tuberculin testing, the values in Table B-1 will represent incidence rather than prevalence, and this incidence is the like-