Long-term follow-up of the QuantiFERON TB-2G test for active tuberculosis disease

To evaluate transitional changes in QuantiFERON TB-2G (QFT-2G) test results in the serial testing on the same patients and to reevaluate the optimal threshold of positive response of QFT-2G test as a cure of TB infection.

We prospectively investigated transitional changes of QFT-2G test results in 22 patients with active tuberculosis (TB) over three years after the initiation of treatment with antituberculosis drugs. Treatment using antituberculosis drugs was performed for six months in all patients.

The positive rate of QFT-2G test results decreased 50% at the treatment completion. Thereafter, although the positive rate of QFT-2G test results has been decreased 45% six months later even if treatment was finished, it decreased slightly to 41% two years later and 36% three years later. If the cut-off value was situated below 50% (IFN-gamma level three years later/ IFN-gamma level of baseline peak value), we could judge the conversion of QFT-2G test in most cases except for two cases three years after the initiation of antituberculosis treatment through this study.

It may be difficult to monitor markers in the cure of TB infection using QFT-2G tests. The cut-off level for a positive response on QFT-2G test may need to be reconsidered when the test is used to monitor the response of active TB to therapy.