Effect of Tuberculin Skin Test on Ex-vivo Interferon-gamma Assay for Latent Tuberculosis Infection

Abstract

Background : Recently, two commercialized whole-blood assays, Gold (QFT) and T (SPOT), which measure the released in the whole blood after being incubation with mycobacterial antigens, were approved for the diagnosis of a latent tuberculosis infection (LTBI). However, there is data on whether or not the previously used PPD skin tests (TST) have any influence on the diagnostic ability of these ex-vivo assays. Methods : Forty-six 15 year-old students who did not appear to be infected with Mycobacterium tuberculosis were enrolled in this study. The peripheral blood was collected and used for two assays. The assays and TST were performed at the baseline (). The TST was repeated two months later (), and the assays were repeated two () and four months () later only in those subjects who had negative results at the baseline in both the assays and TST. An induration size > 10 mm was considered to be positive in the TST. Results : The mean TST value was (range: 0-20). Of the 46 subjects examined, 13 subjects (28.3%) showed positive results in the two-step TST. Nine (19.6%) were SPOT-positive and only one (2.2%) was QFT-positive. The and QFT were carried out in 23 and 25 all-negative subjects, respectively, and all showed negative results. The SPOT was performed in 23 subjects and only one (4.3%) showed a weak-positive result. Conclusion : Even though there were some discrepancies in the results of the two ex-vivo assays, it appears that their results were not influenced by a previous TST carried out in two or four months earlier.