Supplemental use of an interferon-gamma release assay in a state-wide tuberculosis contact tracing program in Victoria: a six-year review.

INTRODUCTION

Tuberculin skin testing (TST) has been the accepted Australian standard for investigating contacts following exposure to infectious tuberculosis (TB). In recent years, the availability of the interferon-gamma release assays (IGRA) has introduced a potential alternative test but data on its use in this context are limited.

METHODS

A prospective longitudinal cohort study was conducted from 2008-2013 to review the use of IGRA and subsequent TB disease following testing in a state-wide contact tracing program. Additional information on the experience and acceptability of IGRA in this context was also obtained through program staff surveys following implementation.

RESULTS

IGRA testing was performed on 643 contacts, with a mean follow-up of 3.7 years. IGRA was primarily used to supplement TST, most commonly due to borderline TST reactivity in individuals who had bacille Calmette-Guérin vaccination. Where both TST and IGRA were performed, correlation of test results was poor (kappa=0.35). The negative predictive value for later development of active TB was 99.5%.

CONCLUSIONS

Our experience suggests that IGRA are able to be incorporated safely and effectively as a supplement to TST-based contact tracing.