تعليقThe Xpert MTB/RIF Ultra assay (hereafter called Xpert Ultra) uses the same GeneXpert platform as the Xpert MTB/RIF test, and was developed to improve the sensitivity and reliability of detection of MTBC and RIF resistance. To address sensitivity, Xpert Ultra uses two multicopy amplification targets (IS6110 and IS1081) and a larger PCR chamber; thus, Xpert Ultra has a lower LoD than Xpert MTB/RIF (16 colony forming units [cfu]/mL and 131 cfu/mL, respectively). At very low bacterial loads, Xpert Ultra can give a “trace” result, which is not based on amplification of the rpoB target and therefore does not give results for RIF susceptibility or resistance. An additional improvement in the Xpert Ultra is that the analysis is based on melting temperature (Tm), which allows for better differentiation of resistance-conferring mutations. Planning the transition to the Xpert Ultra requires special consideration and a GLI document is available to assist in this process (10). WHO recommends using the Xpert Ultra test in the following situations (7):
- In adults with signs and symptoms of pulmonary TB without a prior history of TB or with a remote history of TB treatment (>5 years since end of treatment), Xpert Ultra should be used as the initial diagnostic test for TB and for detection of RIF resistance rather than smear microscopy or culture and phenotypic DST.
- In adults with signs and symptoms of pulmonary TB and a prior history of TB with an end of treatment within the past 5 years, Xpert Ultra may be used as the initial diagnostic test for TB and for detection of RIF resistance rather than smear microscopy or culture and phenotypic DST.
- In children with signs and symptoms of pulmonary TB, Xpert Ultra should be used as the initial diagnostic test for TB rather than smear microscopy or culture in sputum or nasopharyngeal aspirates.
- In adults and children with signs and symptoms of TB meningitis, Xpert Ultra should be used in CSF as an initial diagnostic test for TB meningitis rather than smear microscopy or culture.
- In adults and children with signs and symptoms of extrapulmonary TB, Xpert Ultra may be used in lymph node aspirate and lymph node biopsy as the initial diagnostic test for the detection of lymph node TB, rather than smear microscopy or culture.
- In adults and children with signs and symptoms of extrapulmonary TB, Xpert Ultra should be used for detection of RIF resistance rather than culture and phenotypic DST.
- In adults with signs and symptoms of pulmonary TB who have an Xpert Ultra trace positive result on the initial test, repeated testing with Ultra may not be used.
- In children with signs and symptoms of pulmonary TB in settings with a pretest probability of less than 5% and an Xpert Ultra negative result on the initial test, repeated testing with Xpert Ultra in sputum or nasopharyngeal aspirate specimens may not be used.
- In children with signs and symptoms of pulmonary TB in settings with a pretest probability of 5% or more and an Xpert Ultra negative result on the first initial test, a repeat of one Xpert Ultra test (for a total of two tests) in sputum and nasopharyngeal aspirate specimens may be used.