Comparison of tuberculin skin testing and interferon-gamma release based assay test on children with latent tuberculosis

Objective:Latent tuberculosis infection (LTBI) in childhood causes greater risk of developing tuberculosis disease when compared with older age. Tuberculin skin testing (TST) is utilized extensively in identifying infection with Mycobacterium tuberculosis while the interpretation of this in vivo test is affected by the BCG vaccination status and possible infection with non- tuberculous mycobacteria. In vitro interferon-gamma release based tests are recently introduced to overcome these difficulties. The aim of this study is to compare the results of a interferon-gamma release based assay with the TST in children investigated for LTBI.Material and Methods: Fifty four children between 7 months and 15 years of age were tested with QuantiFERON-TB Gold in Tube (QFT-Gold IT) test based on in vitro interferon- gamma release from peripheral blood T cells in response to Mycobacterium tuberculosis specific antigens ESAT-6, CFP-10 and TB 7.7. Following the Quantiferon assay, the patients were studied in two groups depending on the induration size of their TST, either 10-14 mm and ?15 mm. Isoniazid prophylaxis was started for the patients with ?15 mm tuberculin indurations who were evaluated as LTBI in accordance with contact investigation, clinical and laboratory studies.Results: QFT-Gold IT test was found positive in 9.5% of 21 children with tuberculin induration of 10-14 mm, and in 6% of 33 children with tuberculin induration of ? 15 mm. Isoniazid prophylaxis was given to 64.8% of the patients. In 91.4% of these children, QFT-Gold IT test result was negative. In the group with tuberculin induration sizes of 10-14 mm, that was considered to be due to the BCG vaccination, presence of Mycobacterium tuberculosis infection, therefore the need for prophylaxis could not be excluded by TCT alone in 9.5% , who had positive result in QFT-Gold IT test. In this study, TST positivity and QFT-Gold IT test positivity were found weakly consistent (38.9 %) with kappa value of 0.03(p: 0.63).Conclusion: In a BCG vaccinated population, TST yielded high percentage of induration sizes ?15 mm, probably causing an overdiagnosis of LTBI and unnecessary administration of isoniazid prophylaxis in children. QFT-Gold IT test is a highly specific assay that would provide a solution for such undesirable aspects of TST. Considering the requirement for sophisticated laboratory facilities, in vitro interferon-gamma based tests could still be suggested as supplementary to TST performed with a careful epidemiological and clinical investigation.