Comparison of an in-house and a commercial RDI-based ELISPOT-IFN-γ assay for the diagnosis of Mycobacterium tuberculosis infection

Objective: To compare a RDI-based in-house ELISPOT-interferon-γ (IFN-γ) assay with a commercial (T-SPOT.TB™) assay for the diagnosis of Mycobocterium tuberculosis (TB) infection and the efficacy of the tuberculin skin test (TST) and ELISPOT assay in detecting latent TB infection (LTBI). Design: Eighty-six subjects (65 household contacts of contagious TB-infected patients, 13 subjects with active or previous TB infection, and 8 with suspected TB infection) were consecutively recruited in the context of a surveillance program. Methods: Enrolled subjects underwent the Mantoux TST and two different ELISPOT-IFN-γ assays: an in-house assay using a pool of selected M. tuberculosis peptides (MTP) and the commercial T-SPOT.TB assay. Results: The in-house and commercial ELISPOT-IFN-γ assays showed almost complete concordance (99%) in diagnosing acute or LTBI. When comparing the efficacy of the TST with the in-house ELISPOT assay in detecting TB infection, a small agreement was observed (k=0.344, P0.0001): 36% of the subjects with a positive TST were ELISPOT-MTP negative and 12% with a negative TST were ELISPOT-MTP positive. Furthermore, 78% of the ELISPOT-MTP negative individuals were ELISPOT- Bacillus Calmette-Guérin (BCG) positive, most of whom had received BCG vaccination. Conclusion: Our in-house ELISPOT assay based on a restricted pool of highly selected peptides is equivalent to the commercial T-SPOT.TB assay, is cheaper and is probably not confounded, unlike the TST, by BCG vaccination in our setting.