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

Lee, Jung Yeon; Choi, Hee Jin; Cho, Sang Nae; Park, I Nae; Oh, Yeon Mok; Lee, Sang Do; Kim, Woo Sung; Kim, Dong Soon; Kim, Won Dong; Shim, Tae Sun

Abstract

BACKGROUND
Recently, two commercialized whole-blood assays, QuantiFERON(R)-TB Gold (QFT) and T SPOT-TB(R)(SPOT), which measure the IFN-gamma 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 IFN-gamma assays. METHOD: 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 IFN-gamma assays. The IFN-gamma assays and TST were performed at the baseline (1st). The TST was repeated two months later (2nd), and the IFN-gamma assays were repeated two (2nd) and four months (3rd) later only in those subjects who had negative results at the baseline in both the IFN-gamma assays and TST. An induration size > 10 mm was considered to be positive in the TST.
RESULTS
The mean TST value was 3.1 +/- 5.4 mm (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 2nd and 3rd QFT were carried out in 23 and 25 all-negative subjects, respectively, and all showed negative results. The 2nd 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 IFN-gamma assays, it appears that their results were not influenced by a previous TST carried out in two or four months earlier.