OBJECTIVE:To compare PPD conversion with γ-interferon release assay (IGRA) in determining the newly Mycobacterium tuberculosis (MTB) infection during a TB outbreak.METHODS:The 505 subjects exposed to a TB outbreak were divided into 3 groups based upon the induration diameters of PPD before the outbreak. The changes of PPD induration diameters were observed in different groups, and correlation between PPD conversion or IGRA and exposure levels were analyzed by logistic regression.RESULTS:In subjects with the highest exposure level, the increase in induration of PPD was (12 ± 5) mm, (7 ± 4) mm, and (5 ± 3) mm respectively among the previous PPD < 5 mm, 5 - < 10 mm, and ≥ 10 mm groups, the difference being significant among groups, χ(2) = 43.12, P < 0.01. Using logistic regression analysis, PPD conversion was related to the exposure level (OR = 4.70, P < 0.05) only in the PPD < 5 mm group, while IGRA positivity was closely related to the exposure levels in all 3 groups (OR values were 2.16 - 3.60, P < 0.05). In the high exposure group, the subjects with IGRA positivity combined with PPD conversion rate was 26.1% (31/119), and the subjects with IGRA positivity but no PPD conversion rate was 39.5% (47/119), while in the low exposure group the results were 6.5% (12/185) and 16.8% (31/185) respectively, the difference being significant (χ(2) = 22.82 and 19.63, P < 0.01). After 1 year of follow-up, the reversion rate of the subjects with IGRA positivity was 48.1% (91/189), while the mean increase value of PPD was (1.1 ± 4.0) mm.CONCLUSION:IGRA is superior to PPD conversion in the diagnosis of tuberculosis infection, and more valuable in the detection of newly infected cases.

Background: The tuberculin skin test (TST) is commonly used for the diagnosis of latent tuberculosis infection (LTBI) in non-bacille Calmette-Guerin (BCG) vaccination settings. In recent years, attention has been drawn to interferon-gamma release assays (IGRAs), especially in BCG-vaccinated populations. In this study, we evaluated the TST and a new whole blood IGRA in BCG-vaccinated individuals during a tuberculosis (TB) outbreak in China.Methods: A TB outbreak occurred at a university in Dalian, China from March to November 2010. The TST and a whole blood IGRA were used to screen for TB infection. The correlation between exposure levels, TST, and the IGRA were evaluated.Results: We found that agreement between the IGRA and TST was poor (kappa 0.182-0.290). IGRA positivity was associated with the level of exposure, and IGRA positivity and the level of exposure were risk factors for TB incidence. Neither the IGRA nor the TST alone picked up all TB incidences. However, if a 10 mm cutoff for the TST was used in the highest risk exposure group and IGRA positivity was used in the other risk groups, 19 of the 20 (95%) TB cases were identified.Conclusions: A recommended preventive treatment regimen for China should be based on the level of exposure in conjunction with IGRA and TST test results. (C) 2012 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.