Interferon Gamma Assay Useful for Diagnosis of Latent Tuberculosis in Children

NEW YORK (Reuters Health) Sept 19 - Interferon gamma assays can replace tuberculin skin tests for diagnosing latent tuberculosis in children, and particularly in those in whom skin tests results are confounded by previous vaccinations or infections, according to a study from Cambodia.

In the September issue of Epidemiology and Infections, Dr. K. Okada from the Japan Anti-Tuberculosis Association in Tokyo and colleagues point out that unlike the tuberculin skin test, the interferon gamma assay results are not affected by prior Bacille Calmette-Guerin (BCG) vaccination or non-tuberculous mycobacterial infections. The authors also point out that although known to have high sensitivity and specificity in students and adults, these assays have not been well studied in children.

In Phnom Penh City and Battambang Province in Cambodia, the researchers compared the results of tuberculin skin tests and interferon gamma assays (QuantiFERON®-TB Gold) in children aged 5 years or less who were household contacts of adults with tuberculosis. The investigators point out that BCG vaccination is encouraged for all newborns in Cambodia.

Among the 195 children who had both tests, there was a significantly higher positivity rate with the skin test (47 children; 24%) than with the interferon gamma assay (33 children; 17%).
The authors found that "the agreement between the test results was considerable." They add that the odds for positive tests increased with the grades of positive smears and were highest among those with grade 3 positive smear.

The interferon gamma assay will be particularly beneficial among children with false positive TST, as it not influenced by the confounding effects of BCG and non-tuberculosis mycobacteria, Dr. Okada and colleagues believe. "Although logistical as well as economic problems are involved...it will become a strong technology for epidemiological and immunological studies of tuberculosis control, enabling us to estimate the annual risk of infection...and to make clinical diagnoses of pediatric tuberculosis in the developing world as well as in industrialized countries.