[Comparison of interferon-gamma whole blood assay with tuberculin skin test for the diagnosis of tuberculosis infection in tuberculosis contacts]

Abstract

Tuberculin skin test which is used for the detection of latent tuberculosis (TB), has many disadvantages such as false positivities due to cross reactions between environmental mycobacteria and BCG strain, false negativities due to immunosuppression and malpractice, and also difficulties in application and evaluation. Recently a new diagnostic test which measures the production of interferon (IFN)-gamma in whole blood upon stimulation with specific ESAT-6 and CFP-10 antigens of Mycobacterium tuberculosis has been introduced. Since most of the mycobacteria other than tuberculosis and BCG strain do not contain these antigens, the detection of IFN-gamma levels indicates the specific T-cell response against M. tuberculosis. The aim of the study was to compare the tuberculin skin test and whole blood IFN-gamma assay (QuantiFERON-TB Gold, Cellestis Ltd, Carnegie, Victoria, Australia) for the identification of latent TB infection in the contacts with active TB patients. The tests results were evaluated by using Kappa (K) analysis, and K coefficients of < 0.4, 0.4-0.75 and > 0.75 were accepted as poor, moderate and excellent agreements, respectively. A total of 233 subjects from three risk groups were included to the study. Group 1 included the household members (n = 133) who had contact with smear positive index cases, Group 2 included the subjects from community (n = 46) who had contact with smear positive index cases, and Group 3 included health care workers (n = 74) who had contact with TB patients or their specimens. The positivity rates of tuberculin skin test and IFN-gamma assay in the cases were found as 37% and 42%, respectively. There were no significant differences among the three patient groups with regard to the results of the tuberculin skin test (p > 0.05). However, the positive result of the IFN-gamma assay in Group 1 was found statistically higher than the other groups (51.3%, p = 0.013). A poor agreement between the two tests was detected in the results taken from 233 subjects (65.7%, K = 0.28), while agreement was moderate in unvaccinated group (72.7%, K = 0.44). Evaluation of agreement rates of the tests according to the risk groups yielded 64.6% (K = 0.3) for Group 1, 71.7% (K = 0.32) for Group 2, and 63.5% (K = 0.21) for Group 3, which all coefficients showed poor agreement. Although IFN-gamma blood assay has many advantages such as objective and quantitative results, no interference with vaccination due to the use of specific antigens and being practical, the high cost and the need for well-equipped laboratory are its disadvantages. As a result it was concluded that, IFN-gamma blood assay has limited value for the detection of latent TB infection in our country, since the prevalence of TB infection and BCG vaccination rates are high in Turkey.