Recently, two commercialized whole-blood assays, QuantiFERON®-TB Gold (QFT) and T SPOT-TB®(SPOT), which measure the IFN-γ 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-γ assays.

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-γ assays. The IFN-γ assays and TST were performed at the baseline (1^st). The TST was repeated two months later (2^nd), and the IFN-γ assays were repeated two (2^nd) and four months (3^rd) later only in those subjects who had negative results at the baseline in both the IFN-γ assays and TST. An induration size > 10 mm was considered to be positive in the TST.

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 2^nd and 3^rd QFT were carried out in 23 and 25 all-negative subjects, respectively, and all showed negative results. The 2^nd SPOT was performed in 23 subjects and only one (4.3%) showed a weak-positive result.

Even though there were some discrepancies in the results of the two ex-vivo IFN-γ assays, it appears that their results were not influenced by a previous TST carried out in two or four months earlier.