Predictive value of tuberculin induration at 24 h in healthy schoolchildren

Tuberculin (1 TU PPD RT23 Tween 80) was administered to 500 healthy schoolchildren, aged 5-9 years. The induration at 24 h was compared with that at 72 h. The mean (standard deviation) induration of the tuberculin reaction at the end of 24,48 and 72 h was 4.5 (6.75), 4.4 (7.11) and 4.0 (6.91) mm, respectively. When the tuberculin reaction was 0-9 mm, a significant difference was noted between the 24- and 72-h reading (p = 0.0001). There was no difference in the size of the tuberculin reaction between the 24- and 72-h readings when the reaction size was > or = 10 mm (p > 0.05). Considering a tuberculin induration of < 10 mm as negative tuberculin reaction for our reference population in South India, the sensitivity, specificity, positive predictive value and the negative predictive value in our study for the 24-h reading were found to be high. The predictive value of 24-h induration was not affected by age. Reading and interpretation of tuberculin test after 24 h needs due consideration in clinical practice.