The effect of tuberculin skin testing on subsequent interferon-gamma assay results

Currently, the methods used for screening LTBI are TST and IGRA. IGRA is performed after TST in the two-step approach commonly used in certain groups. The aim of the study was to investigate the effect of TST on subsequent IGRA results, and if such an effect is present, to determine the duration of onset and ending of the effect. Forty-four patients who their result of initial IGRA were negative and low TB contact risk underwent TST first and were followed by IGRA on days 3, 7, 30 and 90 after TST. The QuantiFERON Gold Plus test (Qiagen GmbH, Hilden, Germany) was used as the IGRA in the study. The results of days 3, 7, 30, 90 were compared with the results of initial IGRA (day 0). When compared the quantitative IFN-gamma responses noted by day 0 in the both TB1 and TB2 tubes with noted by day 3 (p=0.490, p=0.601), day 30 (p =0.060, p =0.721) and day 90 (p=0.407, p=0.982), it was found no significant difference. But significant boosting effect was observed for TB1 tube (p = 0.028) on the day 7 while there was no significant increase for TB2 tube (p = 0.910). When compared categorical test results for the same days, it was seen that the results of 43 subjects continued to negative for all days. only one of 44 subjects (2.7%) convert to positive on 7th day, it continued positive on 30th day and revert negative on 90th day. The conversion seen in one person was not significant statistically (p = 0.314). In conclusion, an increase in IFN-gamma response was observed in IGRA administered on day 7 after TST. This increase reduced after one month and disappeared after 3 months. but this increase did not affect the categorical test results. It was thought that when using a two-step strategy, it is ideal to perform IGRA within 3 days of performing the TST, however IGST could be performed at later days. Key words: LTBI, TST, IGRA, boosting, conversion.