Interferon-γ-Release Assays Detect Recent Tuberculosis Re-Infection in Elderly Contacts

Abstract

The tuberculin skin test (TST) does not discriminate between recent and remote latent tuberculosis infection (LTBI). This study was carried out to test two interferon-γ (IFN-γ)-based blood assays in recent contacts with high prevalence of remote LTBI. We performed a contact tracing investigation in a nursing home for the elderly, where elderly patients were exposed to a case of pulmonary tuberculosis. TST, QuantiFERON-TB Gold (QFT-G) and T-SPOT.TB (TS.TB) were performed 8 weeks after the end of potential exposure. IFN-γ measurements were recorded and correlation with exposure was evaluated. Twenty-seven (37.5%), 32 (44.4%) and 16 (22.2%) subjects were TST, TS.TB and QFT-G positive, respectively; agreement between TS.TB and QFT-G was good among exposed subjects only (κ=0.915, 0.218 in unexposed, p<0.001). When amounts of IFN-γ were corrected for the number of producing T cells, specific IFN-γ production was significantly different between exposed and unexposed individuals (16.75±5.40 vs 2.33±0.71 IFN-γ IU/1000 SFC, p=0.0001). QFT-G and TS.TB provided discordant results among elderly contacts. Unlike TST, the specific IFN-γ response might discriminate between recent and long-lasting tuberculosis infection.