| Abstract: | Interferon gamma release assays (IGRAs) have been shown to be sensitive and highly specific for the detection of immune memory against Mycobacterium tuberculosis. Little is known about the reproducibility and within-person variability of these assays. Various aspects of short-term reproducibility of a commercial IGRA, the QuantiFERON-TB Gold In-Tube (QFT-IT) assay, were assessed. The QFT-IT assay was performed twice within 3 days in 27 health care workers in Cape Town, South Africa. Two sets of tests were performed by different operators on day 1, and one set was performed on day 3. Aspects such as interoperator, intraoperator, day-to-day variability, and test-retest variability as well as different the storage methods of plasma were investigated. Seventeen of 27 (63%) of participants had at least one positive QFT-IT text; six had discordant results. The agreement of all aspects studied was high, with kappa values between 0.82 and 1.00 for dichotomous measures, and interclass correlations (ICC) of 0.809 to 0.965 were observed for continuous gamma interferon (IFN-γ) measures. The variability of the magnitude of response was highest comparing measures obtained from individuals on different days (ICC of 0.809). The magnitude of the IFN-γ responses between assays performed for individual participants was variable, with ranges from 0.03 to 11 IU/ml, resulting is discordant results for five participants. The results indicate that the QFT-IT assay is a robust and highly reproducible assay. Considerable intraindividual variability occurs in the magnitude of IFN-γ responses, which may influence the interpretation of serial measures.Commercial T-cell-based interferon gamma release assays (IGRAs) have been shown to be sensitive and highly specific for the detection of Mycobacterium tuberculosis infection (19). IGRAs have recently been incorporated into international guidelines for tuberculosis (TB) screening and diagnosis in several countries including in the United States, Canada, United Kingdom, Germany, and France, either as a confirmatory test for a positive tuberculin skin test (TST) or as replacement for the TST (2, 4, 8, 13, 15). It has further been suggested that IGRAs could be used for the serial measurement of gamma interferon (IFN-γ) responses to detect M. tuberculosis infection in high-risk populations such as health care workers and as a tool to monitor the response to treatment in individuals with active TB disease (measured through a decline in IFN-γ responses) (1, 3, 6, 10, 13, 17).Despite the increased use and availability of IGRAs, there are limited published data regarding the reproducibility of the two currently commercial assays, the QuantiFERON-TB Gold In-Tube (QFT-IT; Cellestis, Australia) and the T-SPOT.TB (Oxford Immunotec, United Kingdom) tests. In two recent publications, test-retest variability and within-person reproducibility of the QFT-IT assay were assessed over a period of 12 days and 3 months, respectively, focusing on test agreement, conversions, and reversions (20, 22). Little is known about the short-term within-person variation in T-cell IFN-γ responses. These could be nonspecific but may be important in the interpretation of serial measures and the definition of test conversion and reversion, especially if the risk of intercurrent M. tuberculosis exposure is low (14, 18).In addition to the need for data guiding the interpretation of serial QFT-IT measures, there are additional aspects of the QFT-IT test that require investigation. Although testing of samples by enzyme-linked immunosorbent assay (ELISA) is traditionally performed in duplicate or triplicate, the manufacturers of the QFT-IT assay recommend testing of a single sample per stimulation condition, and limited data are provided regarding test-retest variability. The robustness of these test measures could also be influenced by additional laboratory factors including interoperator and intraoperator variability and storage practices. We conducted a study to investigate the short-term reproducibility of the QFT-IT assay for the detection of M. tuberculosis infection. |
