Comparison of a whole-blood interferon gamma assay with tuberculin skin testing for detecting latent Mycobacterium tuberculosis infection

Context Identifying persons with. latent tuberculosis infection (LTBI) is c rucial to the goal of TB elimination. A whole-blood interferon gamma (IFN-g amma) assay, the Quanti-FERON-TB test, is a promising in vitro diagnostic t est for LTBI that has potential advantages over the tuberculin skin test (T ST). Objectives To compare the IFN-gamma assay with the TST and to identify fact ors associated with discordance between the tests. Design and Setting. Prospective comparison study conducted at 5 university- affiliated sites in the United States between March 1, 1998 and June 30, 19 99. Participants A total of 1226 adults (mean age, 39 years) with varying risks of Mycobacterium tuberculosis infection or documented or suspected active TB, all of whom underwent both the IFN-gamma assay and the TST. Main Outcome Measure Level of agreement between the IFN-gamma assay and the TST. Results Three hundred ninety participants (31.8%) had a positive TST result and 349 (28.5%) had a positive IFN-gamma assay result. Overall agreement b etween the IFN-gamma assay and the TST was 83.1% (kappa =0.60). Multivariat e analysis revealed that the odds of having a positive TST result but negat ive IFN-gamma assay result were 7 times higher for BCG-vaccinated persons c ompared with unvaccinated persons. The IFN-gamma assay provided evidence th at among unvaccinated persons with a positive TST result but negative IFN-g amma assay result, 21.2% were responding to mycobacteria other than M tuber culosis. Conclusions For all study participants, as well as for those being screened for LTBI, the IFN-gamma assay was comparable with the TST in its ability t o detect LTBI, was less affected by BCG vaccination, discriminated response s due to nontuberculous mycobacteria, and avoided variability and subjectiv ity associated with placing and reading the TST.