Although Gene Xpert has shown high overall sensitivity and specficity with pulmonary samples, its sensitivity has been lower with smear-negative pulmonary samples and extrapulmonary samples. In addition, the prediction of rifampin resistance (RIF-R) in paucibacillary samples and for a few rpoB mutations has resulted in both false-positive and false-negative results.
An advanced version of the Gene Xpert MTB/RIF assay with better TB detection capabilities and more definitive identification of RIF susceptibility and resistance was developed, this Gene Xpert Ultra had the following changes
These changes have allowed for an almost 10-fold increase in analytical sensitivity for the detection of M. tuberculosis H37Rv. Importantly, this increase in sensitivity was not restricted only to H37Rv, which contains 16 copies of the IS6110 assay target. Sensitivity was also more than doubled for the detection of BCG, which contains only one IS6110 copy, suggesting that the additional sensitivity gained by Ultra is expected to be true for different clinical M. tuberculosis strains containing both high and low numbers of copies of IS6110.
When Gene XpertUltra was used on 277 sputum samples from TB suspects, the enhanced sensitivity of Ultra led to identification of more cases of smearnegative and culture-positive TB and more TB cases overall. Levels of RIF resistance detection were comparable between Xpert and Ultra, although Ultra detected a hetero-resistant sample that was missed by both phenotypic susceptibility testing and Xpert. Analytic results have con?rmed that Ultra has an improved ability to detect resistance in mixed samples, an observation that likely explains why Ultra detected RIF-R in the hetero-resistant clinical sample. Importantly, the manual steps required to perform Xpert and Ultra are identical. Furthermore, the two assays can be run in identical GeneXpert instruments after a software upgrade. Thus, it is expected that Ultra can be implemented with little additional training in sites that already use the current Xpert assay.
Results suggest that Ultra will result in greater TB case detection rates not only in subjects with paucibacillary TB, such as those with HIV coinfection or tubercular meningitis, but also in pediatric patients with TB and those with extrapulmonary TB, which are known to have lower mycobacterial loads. As TB elimination programs achieve initial successes, rates of paucibacillary TB should increase compared to rates of smear-positive disease. In this setting, assays with increased sensitivity, such as Ultra, may be instrumental in identifying the remaining cases of TB, and these assays are likely to prove valuable in furthering WHO goals to eradicate this disease.