Clinical utility of a T cell-based assay in the diagnosis of extrapulmonary tuberculosis

The aim of this study was to evaluate the QuantiFERON TB-2G (QFT-2G) results in patients with active extrapulmonary tuberculosis (E-TB) to determine whether QFT-2G test might be a reliable diagnostic method for detecting active E-TB infection compared with the tuberculin skin test (TST).

TST and QFT-2G tests were performed for 35 patients with confirmed E-TB, 30 with probable E-TB and 45 who did not have E-TB.

For patients with confirmed E-TB, the positive rate of the TST was 57% and that of the QFT-2G test was 86%. For patients with probable E-TB, the positive rate of the TST was 60%, whereas that of the QFT-2G test was 80%. However, the positive rate of the TST was 51% and that of QFT-2G test was 9% for patients that did not have E-TB. The QFT-2G test showed a significantly lower percentage of false-negative results compared with the TST. The sensitivity and specificity of the TST for the diagnosis of active E-TB were 57% and 49%, respectively. By comparison, the sensitivity and specificity of the QFT-2G test were 86% and 84%, respectively. When the results of the TST and QFT-2G tests were separated by the site of E-TB, the positive rates for both tests in patients with miliary tuberculosis were lower than those in patients with other E-TB disease, although the difference was not significant.

The QFT-2G appears to be a reliable diagnostic test and in the appropriate clinical context QFT-2G may be more useful than the TST to support a diagnosis of E-TB. Studies are needed to evaluate its value also in situations of low clinical probability.