Concordance Between the Tuberculin Skin Test and Interferon-Gamma Release Assay in Uveitis Patients in India

Abstract

Purpose: : The utility of the IFN-γ assay, a new screening test for latent tuberculosis infection (LTBI), has not been studied in a uveitis population in South India. The purpose of this study was to compare the concordance between the 1 Tuberculin Unit (TU) tuberculin skin test (TST), 5 TU TST, and the QuantiFERON- TB Gold In-Tube Test (IFN-γ assay) in this group of patients.

Methods: : In this prospective cohort study of 122 consecutive patients seen in the Uveitis Clinic at Aravind Eye Hospital in Madurai, India, 73 patients received 1-TU TST and 47 received 5-TU TST. Millimeters of induration were read 48-72 hours later, and ≥ 10mm of induration was considered positive. All patients had the IFN-γ assay. UCSF and Aravind IRB approval were obtained.

Results: : Of the 122 enrolled patients, 73 (59.8%) were male and 49 (40.2%) female. 2 (1.6%) patients were excluded from the 1-TU dose group for non-reproducible IFN-γ assay results. 2 (1.6%) patients were excluded from the 5-TU dose group for failure to return for the TST reading. 115 (97.5%) patients had BCG scars. Twenty-two (31.0%) of 71 patients in the 1-TU group were negative for the TST & IFN-γ assay. 5 (7.0%) were positive for TST and negative for IFN-γ assay. 37 (52.1%) were negative for TST and positive for IFN-γ assay. 7 (9.9%) were positive for the TST & IFN-γ assay. 10 (21.3%) of the 47 patients in the 5-TU group were negative for the TST & IFN-γ assay. 14 (29.8%) were positive for the TST and negative for the IFN-γ assay. 3 (6.4%) were negative for the TST and positive for the IFN-γ assay. 20 (42.6%) were positive for TST & IFN-γ assay. The 1-TU TST results were discordant with the IFN-γ assay, with a correlation coefficient of -0.03, 95% CI -0.27 to 0.20 (P = 0.78). The 5-TU results revealed a significant correlation of 0.32, 95% CI 0.036 to 0.556 (P = 0.03).

Conclusions: : Recent publications indicate that the IFN-γ assay has higher sensitivity and specificity than TST for LTBI. We found no correlation between the 1-TU TST and the IFN-γ assay, indicating that the currently used 1-TU dose in India may miss cases of LTBI and possible ocular TB. Although significant, the moderate correlation between the 5-TU dose and IFN-γ assay results suggests the IFN-γ assay may provide additional information beyond TST in the diagnosis of LTBI in the South Indian uveitis population.

Keywords: uveitis-clinical/animal model • detection • bacterial disease