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Wednesday 16 January 2019
Diagnostic performances of the Xpert MTB/RIF in Brazil

This week we suggest to read "Diagnostic performances of the Xpert MTB/RIF in Brazil", an article published on "Respiratory Medicine", volume 134, in January 2018. 


Tuberculosis (TB) is a first-class health priority, with over 10.4 million cases notified in 2015, of whom 480,000 are affected by multidrug-resistant (MDR) form and additional 100,000 have rifampicin-resistant TB (RR-TB) following diagnosis with Xpert MTB/RIF. In Brazil, 84,000 TB incident cases were estimated to occur in 2015, with 1900 MDR- and RR TB cases. Xpert MTB/RIF has changed the programmatic approach to TB diagnosis, allowing the clinician to know in less than 2 hours if the case is affected by TB and if there is resistance to rifampicin. This is particularly true in high incidence TB countries, where the sensitivity and specificity of the test are high. The test was endorsed by World Health Organization (WHO) in 2011. However, the sensitivity of Xpert MTB/RIF is higher in sputum smear positive than in negative cases. The positive and negative predictive values of the test critically depend on the prevalence of TB. In countries where the incidence of TB is intermediate, like in Brazil, information on Xpert MTB/RIF is lacking.

The authors reported a study designed to evaluate the diagnostic performances of the test in the HIV- uninfected and – infected patients, within the framework of the ERS (European Respiratory Society)/SBPT (Brazilian Society of Respiratory Diseases) collaborative TB project.

407 patients with culture-confirmed TB were retrospectively enrolled in an outpatient TB clinic, located in Alvorada – Rio Grande do Sul, from January 2015 to December 2016. No specific selection criteria were adopted. Paediatric cases were excluded.

The majority was male (238, 58.5%), white (313/407, 76.9%) with a median age of 54 years. More than 70% of the cases were current or former smokers and 23.9% had a previous TB episode. The proportion of HIV- infected patients was higher than 10%. More than 60% did not show radiological abnormalities at the chest radiography. Only 1.0% of the patients were resistant to rifampicin. Treatment outcome was described for 159 patients: 70.4% were cured, 17.6% defaulted, 7.6% failed, and 4.4% died.

The Xpert MTB/RIF and the culture were performed in all 407 patients. Using culture as the gold standard, the diagnostic performance of the Xpert MTB/RIF in the HIV-negative patients was as follows: sensitivity 100.0%, specificity 92.8%; positive and negative predictive values were 71.4% and 100.0%, respectively. In the HIV- infected subgroup the specificity was significantly lower (59.3%), probably due to the relatively small sample size and the immunosuppression-related lower sputum smear positivity rate. Out of 108 Xpert positive TB cases, 33 (30.6%) were culture negative: they all were likely to have TB, and 11/33 (33.3%) were HIV infected. The multivariate logistic regression analysis found that a true positivity is associated with increasing age.

The authors concluded, highlighting the importance of their study as the first describing the association between age and positive Xpert test in an adult population, and investigating the performances of the test in a country with an intermediate TB incidence, revealing a very high sensitivity and specificity in HIV-negative patients,  underlining its importance to ensure quality-assured diagnosis and effective treatment, which are core activities to control TB.

AUTHORSDenise Rossato Silva, Giovanni Sotgiu, Lia D'Ambrosio, Giovana Rodrigues Pereira, Márcia Silva Barbosa, Natan José Dutra DiasLaura Saderi, Rosella Centis, Giovanni Battista Migliori

Read more here.