Value of dual testing for identifying tuberculous infection.

Radhakrishna, S.; Frieden, T.R.; Subramani, R.; Narayanan, P.R.

Tuberculosis; 2006; 86; 47–53.

Summary: Setting: A rural community in Chingleput district in Tamil Nadu state in south India.

Objective: To determine the value of dual testing with PPD-S and PPD-B for identifying subjects with a tuberculous infection.

Design: About 240,000 subjects in rural south India, all of whom were tested initially with PPD-S and PPD-B, were followed up for 15 years, mainly by total population survey once in every 2 ½ years. The incidence of culture-positive tuberculosis was estimated using life-table technique.

Results: Among 17,530 subjects with an intermediate reaction (8–11 mm) to PPD-S at intake, 285 with an induration to PPD-S exceeding the induration to PPD-B by at least 2 mm, had a significantly higher incidence of culture-positive tuberculosis than the remaining (154 and 93 per 100,000), and similarly 481 who had an induration of <10mm to PPD-B compared to those with > 10mm (131 and 93 per 100,000). These subjects may be regarded as having a tuberculous infection. Infection with non-tuberculous mycobacteria conferred protection of about 30% against the development of tuberculosis over a 15-year period.

Conclusion: In subjects with an intermediate reaction (8–11 mm) to PPD-S, dual testing with PPD-B enabled identification of those with a tuberculous infection. Most of the reactions were due to non-tuberculous mycobacteria.

Keywords: Dual testing; PPD-S; PPD-B; Identifying infected

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