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Tuberculosis in the Workplace
TABLE B-1. Predictive Value of a Positive Tuberculin Test
Predictive Value (%) at Indicated Specificity
Prevalence of Infection (%)
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).
THE BOOSTER PHENOMENON
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.
Implications of Operating Characteristics of Tuberculin Test for Serial Skin Testing in Medical Populations
In considering serial tuberculin testing, the values in Table B-1 will represent incidence rather than prevalence, and this incidence is the like-