Our goal in the present study was to examine the effect of standard LTBI treatment on QFT-G assay results in a US occupational health setting in which isoniazid treatment compliance was monitored and reexposure to active TB was not a confounding factor. Our study design reflects the fact that the QFT-G assay has been sanctioned by the Centers for Disease Control and Prevention for use in HCW screening programs.1
All 5 newly hired HCWs who initially tested positive by the QFT-G assay on routine screening and who then received 9 months of closely monitored isoniazid therapy continued to have positive QFT-G results at the time of treatment completion; 3 of these HCWs had repeat QFT-G tests performed 3 months later, and results for all 3 HCWs were still positive.
Previous studies of the effect of LTBI therapy on IGRA results6–11
have generated conflicting data, with some studies suggesting that responses to TB-specific antigens might decrease sufficiently with treatment that IGRAs might provide a useful means of monitoring treatment efficacy, and other studies suggesting that responses might not decrease sufficiently for IGRAs to be used for this purpose. However, the comparability of these studies to each other and to our study is confounded by multiple factors, including repeated exposure of individuals to active TB during and after LTBI treatment,9
differences in LTBI treatment protocols, compared with the current US standard,6–8,10,11
and differences in study populations (eg, studies of recent contacts).7,8,10,11
Our study uniquely assesses the impact of the US standard LTBI treatment course on QFT-G test results in newly hired HCWs who have received a diagnosis of LTBI by routine hospital screening protocols; the majority of these HCWs were likely to have been remotely infected.
Although our sample size is small, our results unequivocally indicate that QFT-G test results should not be used to assess the effectiveness of recent or remote treatment courses for LTBI: the majority of individuals who initially test positive by the QFT-G assay will continue to test positive after 9 months of isoniazid treatment. This result has significant implications for HCW screening programs, because it should not be assumed that HCWs who report prior LTBI therapy but who still test positive by the QFT-G assay have not received appropriate treatment in the past. Moreover, neither physicians nor patients should expect reliable changes in QFT-G test results after standard treatment. Finally, our results neither support nor refute the idea that a positive IGRA result, in the absence of treatment for LTBI, is associated with a higher risk of reactivation of LTBI, compared with a positive tuberculin skin test result alone. Only long-term follow-up of large numbers of persons will be able to address this topic.