[Evaluation of tuberculin skin test conversion and interferon-gamma release assay in diagnosis of tuberculosis infection].

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.