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IFN- Release Assays vs. TST for Predicting Development of Active TB

Among individuals exposed to TB, 14.6% of those with a positive IFN- release assay developed active TB, compared with only 2.3% of those with a positive tuberculin skin test.

In areas with a low prevalence of TB, the cornerstone of control is to identify individuals with latent infection. Traditionally, the tuberculin skin test (TST) has been used for this purpose, but in vitro interferon (IFN)- release assays are also now available.

To compare these two testing approaches, researchers conducted a longitudinal cohort study in Hamburg, Germany, among 601 individuals with recent TB exposure. Each study participant was tested with both the TST and the QuantiFERON-TB Gold In-Tube (QFT) IFN- release assay at least 8 weeks after exposure. TSTs were scored as positive if indurations were >5 mm. Isoniazid chemopreventive therapy was offered only to individuals with positive QFT results.

During the 2-year study, active TB was documented in 6 of 41 (14.6%) subjects who had positive QFT results but who refused isoniazid therapy, compared with 5 of 219 (2.3%) subjects who had positive TSTs only (P<0.003).

Comment: The proportion of individuals who developed active TB was significantly higher among people who had positive QFT results than among those with positive TSTs only. However, this difference might have been smaller if the cutoff for a positive TST had been higher (e.g., 10-mm induration). In addition, this study population had a high rate of BCG vaccination, which confounded the TST results. In the U.S., where BCG vaccination is not used, cross-reactivity would be far less of a problem, and the difference between QFT and TST results might not be so striking.

— Neil M. Ampel, MD

Published in Journal Watch Infectious Diseases May 28, 2008

Citation(s):

Diel R et al. Predictive value of a whole blood IFN- assay for the development of active tuberculosis disease after recent infection with Mycobacterium tuberculosis. Am J Respir Crit Care Med 2008 May 15; 177:1164.

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