Objective: To evaluate the effect of neonatal vaccination with bacille Calmette-Guérin (BCG) on tuberculin skin test (TST) reactivity over time and to define the optimal age-specific induration cut-offs to detect latent tuberculosis infection (LTBI).
Design: TSTs were performed on 783 children aged 3 months to 14 years who had received neonatal BCG. The estimated annual risk of LTBI was derived from TSTs administered to 2504 children aged 7 years who lacked BCG scars. Goodness-of-fit analysis was used to determine the optimal age-specific cut-off values.
Results: The effect of neonatal BCG on TST induration waned with age, reaching a nadir at age 6-7 years. This was followed by a rise in TST reactivity. The optimal age-specific TST cut-off values for the detection of LTBI was estimated to be respectively 21, 18, 13 and 10 mm at ages 0-1, 2-3, 4-5 and 6-7 years. There was a close correlation between these new cut-off values with the estimated risk of LTBI for the first 7 years of life (r = 0.93, P < 0.001).
Conclusions: The effect of neonatal BCG on TST gradually declines over the first 7 years of life. Our proposed new age-specific TST induration cut-off values could help differentiate between response to BCG and LTBI in young children.