Tuberculin test conversion during repeated skin testing, associated with sensitivity to nontuberculous mycobacteria

To determine whether repeating the tuberculin test after a brief interval might result in tuberculin conversion, the authors tested 213 healthy volunteers twice, 1 month apart, with 5 TU of tuberculin purified protein derivative (PPD). Three nontuberculous mycobacterial antigens (PPD-G, PPD-Y, and PPD-B) were also applied with the first tuberculin test. By widely used criteria, 14 volunteers (6.6 per cent) converted their tuberculin tests from negative to positive on the second testing. Whereas 13 of 103 subjects (12.6 per cent) with nontuberculous antigen sensitivity converted their tuberculin test to positive, only one of 110 subjects (0.9 per cent) with no known prior mycobacterial sensitivity converted to positive (P<0.005; x ²=10.05). When retested with 5 TU of tuberculin PPD 6.5 months after the second test, nine of 13 converters reverted to negative. The authors conclude that tuberculin conversion may occur when the skin test is repeated at 1 month, and that boosting of cross-reacting mycobacterial sensitivity might have caused a portion of the conversions in this population of young, healthy midwestern volunteers. Sensitivity to Mycobacterium tuberculosis might also be boosted by tuberculin testing. Because the prevalence of sensitization by tuberculous and nontuberculous mycobacteria can be expected to vary in different populations, the significance of tuberculin conversion will also vary with the population being tested.