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The effect of previous tuberculin skin test on the follow up exam of whole blood interferon-g assay in the screening for latent TB infection

Division of Pulmonary and Critical Care Medicine, Department of Medicine (Drs J. C. Choi, Shin, Kim, Park, B. W. Choi), Department of Laboratory Medicine (Dr Lee) Chung-Ang University School of Medicine, Seoul, Korea

bwchoimd{at}cau.ac.kr

Abstract

Background: A two-step procedure using a tuberculin skin test (TST) followed by an IFN-g assay in cases where the TST is positive has been advocated to screen for latent tuberculosis infection. However, TST could also boost the in-vitro immune response. In this study, we evaluated the effect of TST on the results of the IFN-g assay.

Methods: Our study included 84 health care workers who were working in the department of pulmonary medicine more than 1 year. First, whole blood IFN-g assay was performed and then the TST was applied. After 2-4 weeks later, a follow up IFN-g assay was performed. QuantiFERON-TB GOLD was used for the IFN-g assay.

Results: Valid TST results were available in 82 individuals because two participants refused to take the TST after the IFN-g assay. The TST was positive in 36 out of 82 participants (42.7%) and the IFN-g assay was positive in 16 out of 82 participants (19.5%). The overall agreement between the two tests was 67.5% (k=0.31; 95% CI, 0.22-0.40). The IFN-g levels increased significantly from 0.05 to 0.19 (p = 0.011) and 3 out of 18 participants (16.7%) had conversion of their IFN-g assay results in the TST positive group. However, in the TST negative group, the IFN-g levels did not change after the TST.

Conclusion: The agreement between the TST and the IFN-g assay was low, and IFN-g level could be influenced by the TST, in the TST positive population, when a follow up IFN-g assay is performed 2-4 weeks later.

Key Words: latent tuberculosis • interferon-g assay • tuberculin skin test