Researchers validate a highly sensitive molecular test to detect cases of TB
The results confirm that the ‘Xpert Ultra’ is a frontline tool in routine conditions and is effective in different contexts
The ‘Xpert Ultra’ molecular test has a greater capacity than its predecessor (‘Xpert MTB/RIF’) in detecting tuberculosis cases, either passively (i.e. people who attend the hospital with disease symptoms) or actively (searching for possible cases in the community among contacts of cases). This is the main conclusion of a study performed by ISGlobal, an institution supported by “la Caixa” Foundation, in collaboration with the Manhiça Health Research Centre (CISM), published in the European Respiratory Journal.
Tuberculosis (TB) is the leading cause of death by an infectious
agent, worldwide. In 2019, 1.4 million people are estimated to
have died and 10 million people fell sick from TB, although only
70% of the cases were diagnosed.
“Most national
programmes for TB control focus on detecting cases that attend
the hospital, which tend to be more severe, but there are many
cases with no or few symptoms that we are missing,” says
Alberto García-Basteiro, ISGlobal and CISM researcher and
last author of the study. “If we want to achieve the End
TB goals, we need to develop new diagnostic tools capable of
identifying patients at earlier stages of the disease, with
lower bacterial burdens, and that can be deployed at the
patient’s point of care,” he adds.
The
team led by García-Basteiro performed a field study to
compare the performance of two molecular tests: Xpert, developed
in2010, and its improved version, Xpert Ultra, developed three
years ago and capable of detecting smaller amounts of DNA. They
used the same sputum sample to compare both tests, and performed
liquid bacterial cultures as reference for both tests. The study
was conducted in the distric of Manhiça, a region with
high TB and HIV prevalence, on two cohorts: one of patients
attending healthcare services and having symptoms compatible
with TB, and another cohort from the Xpatial-TB study, which
performed an active search for cases with TB in the same
district.
The results show that, in the cohort of
patients voluntarily attendig health centers, (around 1,400
patients), the Ultra test was considerably more sensitive thatn
the Xpert (i.e. it detected more cases), although its
specificity was slightly lower. Among the cohort of case
contacts, the incidence was much lower but even then the Ultra
managed to detect cases that were not detected by Xpert or by
liquid culture, likely because they had a very low burden of
bacteria. The specificity of both tests was similar in this
setting.
“This study is the largest to date
comparing the performance of Xpert Ultra and Xpert with a same
sputum sample coming either from routine diagnostic activities
or from active case finding,” says Belén Saavedra,
microbiologist at ISGlobal and first author of the study.
“The Ultra test can help us identify asymptomatic cases in
early stages of the disease, which would lead to more effective
actions to interrupt transmission in the community,” she
adds. The authors conclude that the Ultra test is a frontline
tool for TB diagnosis in different settings.
Reference
Saavedra B, Mambuque E, Nguenha D et al. Performance of Xpert MTB/RIF Ultra for tuberculosis diagnosis in the context of passive and active case finding. Eur Resp J. 2021. DOI: 10.1183/13993003.00257-2021
