Annual risk of tuberculous infection among schoolchildren in Bhutan

School-based survey in the mountainous nation of Bhutan.

To estimate the annual risk of tuberculous infection (ARTI) among children aged 6-8 years.

A national-level tuberculin survey was carried out among children attending 64 schools selected by two-stage cluster sampling. The study population was comprised of children without and with bacille Calmette-Guérin (BCG) scar. Tuberculin testing was performed using 2 tuberculin units of purified protein derivative RT23. The maximum transverse diameter of induration was measured at 48-72 h.

Of 6087 satisfactorily test-read children, 82% had a BCG scar. The frequency distribution of tuberculin reaction sizes in all children (with and without BCG scar) did not reveal the mode for tuberculous reactions. The mode seen at 17 mm among children without BCG scar was applied to estimate the prevalence of infection among all children using the mirror-image method. Estimation was also undertaken by shifting the mode by 1 mm on either side. The ARTI computed from the prevalence thus estimated varied between 0.2% and 0.7%. There was no difference in the prevalence of infection by BCG scar status, implying that the estimated ARTI was not influenced by BCG-induced tuberculin sensitivity.

The ARTI has declined in Bhutan compared to the 1991 survey estimate of 1.9%.