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The risk of latent tuberculosis (LTBI) in healthcare workers (HCWs) is high. Until recently, the tuberculin skin test (TST) was the only diagnostic test available for Mycobacterium tuberculosis infection. A new enzyme-linked immunosorbent assay test, the QuantiFERON-TB Gold (QFT-G) test, was recommended by the US Centers for Disease Control and Prevention as an alternative test for the diagnosis of LTBI in HCWs. The purpose of our study was to compare the TST and the QFT-G test in Spanish HCWs in order to improve procedures for the detection of LTBI.
A cross-sectional study with blinded comparison of TST and QFT-G test results was carried out among 134 HCWs at an 800-bed Spanish university hospital. The level of interferon-7 production stimulated by the QFT-G test was measured. A concentration of at least 0.35 IU/mL was considered a positive result. An induration of at least 5 mm in non-BCG-vaccinated or at least 15 mm in BCG-vaccinated HCWs for the TST was considered positive.
Of the 134 HCWs included (mean age, 33.4 years; 101 [75.4%] women; 47 [35.1%] BCG vaccinated), the LTBI prevalence diagnosed with any test was 11.2% (95% confidence interval, 6.6%-18.1%), with the TST was 8.96%, and with the QFT-G test was 5.97% (nonsignificant differences). The QFT-G test value was higher in subjects with TST induration of at least 15 mm than in subjects with TST induration of less than 15 mm (P < .001). Overall agreement between the results of the two tests was found in 94% of HCWs (K = 0.56), but agreement was only 59% in HCWs who had a positive result for both tests. Disagreement was present in the results found for 5% of HCWs.
Few studies have compared both tests in populations with high M. tuberculosis exposure but low BCG vaccination prevalence. Agreement between both tests is high, especially among negative Results. Studies are needed to clarify the reasons for disagreement and to establish the best TST and QFT-G test cutoff point.