To analyse the distribution of tuberculin skin test (TST) reactions and epidemiological factors related to TST reactivity.
DESIGN: TST reactivity was analysed in 1190 students. A linear regression model was created for the relative contribution of background factors of TST reactivity. A subgroup of 287 non-vaccinated subjects was comparatively skin-tested with Mycobacterium avium sensitin and tuberculin.
RESULTS: Among non-bacille Calmette-Guérin (BCG) vaccinated students, 91% had no TST reaction (0 mm induration) and reactions of ≥ 10 mm were found in 2.9%, whereas 34% of BCG-vaccinated students had no TST reaction and 42% had reactions of ≥ 10 mm. The expected contribution to TST reactivity was 6.0 mm for a history of BCG vaccination, 3.0 mm for a country of birth with medium/high incidence of TB and 1.6 mm per 10 years of age. The sensitin reactions exceeded the TST reactions by ≥ 3 mm in 52% of the comparatively tested subjects with TST reactions of ≥ 1 mm.
CONCLUSION: BCG vaccination, cross-reactivity with non-tuberculous mycobacteria, geographic origin and age had a decisive influence on TST reactivity. Most non-vaccinated health care students were non-reactive, which highlights the need to organise preventive measures in settings where TB exposure is expected.
Authors: Fjällbrant H, Rutqvist A, Widström O, Zetterberg G, Ridell M, Larsson LO
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