Interferon-Gamma Release Assay for the Diagnosis of Latent TB Infection – Analysis of Discordant Results, when Compared to the Tuberculin Skin Test

BACKGROUND With the Interferon-gamma release assays (IGRA) a new method for the diagnosis of latent tuberculosis infections (LTBI) is available. Due to the lack of a gold standard for the diagnosis of LTBI, the IGRA is compared to the Mantoux Tuberculin Skin Test (TST), which yields discordant results in varying numbers. Therefore we assessed to which extent discordant results can be explained by potential risk factors such as age, BCG vaccination and migration. METHODS AND FINDINGS In this pooled analysis, two German studies evaluating the Quantiferon-Gold In-Tube test (QFT) by comparison with the TST (RT23 of SSI) were combined and logistic regressions for potential risk factors for TST+/QFT- as well as THT-/QFT+ discordance were calculated. The analysis comprises 1,033 participants. Discordant results were observed in 15.4%, most of them being TST+/QFT- combinations. BCG vaccination or migration explained 85.1% of all TST+/QFT- discordance. Age explained 49.1% of all TST-/QFT+ discordance. Agreement between the two tests was 95.6% in German-born persons younger than 40 years and not BCG-vaccinated. CONCLUSIONS After adjustment for potential risk factors for positive or negative TST results, agreement of QFT and TST is excellent with little potential that the TST is more likely to detect old infections than the QFT. In surveillance programs for LTBI in high-income, low TB incidence countries like Germany the QFT is especially suited for persons with BCG vaccination or migrants due to better specificity and in older persons due to its superior sensitivity.