Within-Subject Variability of Mycobacterium tuberculosis-Specific Gamma Interferon Responses in German Health Care Workers

Abstract

Gamma interferon (IFN-␥) release assays (IGRAs) are used increasingly for the periodic tuberculosis (TB) screening of health care workers (HCWs), although data regarding the reproducibility and interpretation of serial testing results in countries with a low incidence of TB are scarce. The present study evaluated and compared the within-subject variability of dichotomous and continuous results of two commercial IGRAs, the QuantiFERON-TB Gold In-Tube (QFT) and the T-SPOT.TB (T-SPOT), in German HCWs during a 4-week period. Thirty-five immunocompetent HCWs with low or medium TB screening risk and without known recent TB exposure or tuberculin skin test application were tested repeatedly with both IGRAs at weekly intervals. A total of 158 valid results were obtained for each IGRA. Changes of about ؎70% (QFT) and ؎60% (T-SPOT) from the mean IFN-␥ response accounted for 95% of the within-subject variability. However, according to the manufacturers' cutoffs, inconsistent results were observed more frequently for the QFT (28.6%; four conversions, six reversions) than for the T-SPOT (8.6%; three reversions; P < 0.001). The overall agreement between the IGRAs was good. Regression toward the means accounted for a significant decline in mean IFN-␥ responses of about 25% between successive visits for both IGRAs. Although both assays were highly reliable and reproducible, we observed substantial within-subject variability and regression toward the means during a 4-week period, which should be considered when interpreting serial testing results in comparable populations and settings. Our data support the use of borderline zones for the interpretation of serial IGRA results and the retesting of borderline positive results before offering preventive chemotherapy.

Reproducibility of Interferon Gamma (IFN-γ) Release Assays. A Systematic Review

“…Overall, the ICC was high (0.99) despite the high rate of conversions and reversions seen. In the two studies for which the quantitative data were not available from authors, an overall discordant rate of 4.8% on 145 subjects was reported in one study (27) and no discordance in the other (35 subjects) (22).…”

“…Simply repeating optical density measurements of the ELISA test result after a time interval (up to 2 h) did not result in any variability (22,30). Three other studies, for which quantitative data were not available, assessed additional sources of QuantiFERON variability: diurnal variability, interlaboratory variation, and variability comparing the use of an automated versus manual ELISA (29,38,39).…”

“…A total of seven studies were included (6,(22)(23)(24)(25)(26)(27). Individual test data were available for five studies with a total of 739 subjects, with each subject providing two test results obtained in identical settings.…”

“…A total of three studies used T-SPOT (6,22,30) and assessed variability under the same conditions (125 subjects) on repeat testing over time (270 subjects) and boosting by negative skin test (63 subjects). Percent of conversions and reversions are summarized in Table 4; variation of results on repeat testing were not reported if the baseline test was in the borderline range (4.1-7.9 spot-forming units).…”

IFN-γ release assay versus tuberculin skin test for monitoring TB infection in healthcare workers

“…However, the effect, if any, seems to be on a minor scale [20]. Reasons for test variability of IGRA may be based on variations of the handling and reading of the tests, variability of the immune system independent of infection status, and differences in the activity of the M. tuberculosis infection, that is, transient infection, low replication of M. tuberculosis with no stimulation of the immune system, high replication of M. tuberculosis with stimulation of the immune system and uncontrolled replication causing active TB [75,76]. In addition, reversions are partly explained by the statistical mechanism of regression toward the population mean [20].…”