Absence of interferon-gamma release assay conversion following tuberculin skin testing

The tuberculin skin test (TST) has been the established screening method for tuberculosis (TB) for over a century. Interferon-gamma release assays (IGRAs) using Mycobacterium tuberculosis-specific antigens are increasingly used as diagnostic tests for TB. Tuberculin comprises multiple antigens, including the antigens used in the QuantiFERON-TB Gold In-Tube (QFT-GIT) assay. Exposure to these antigens by means of a TST may prime an immune response that leads to a false-positive result in a subsequent IGRA, limiting the validity of IGRAs in patients in whom these tests are performed sequentially. The current data on the influence of prior TST on IGRAs show inconsistent results.

Sixteen non-bacille Calmette-Guérin immunised medical students with no history of TB exposure and minimal risk of exposure to TB during the study period were tested simultaneously with a TST and QFT-GIT. The QFT-GIT assay was repeated 6 and 10 weeks later.

At baseline, all TST and QFT-GIT results were negative and remained negative 6 and 10 weeks after the TST.

These data show that negative QFT-GIT results are reproducible and suggest that a TST does not result in conversion of subsequent QFT-GIT assays in the absence of concomitant TB exposure. Therefore, a positive QFT-GIT should not be attributed to boosting induced by a previous TST.