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Rapid Diagnostic Test Evaluated for Tuberculosis

By Labmedica International staff writers
Posted on 26 Apr 2018
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Image: The Xpert MTB/RIF assay loaded into the GeneXpert 4-module machine (Photo courtesy of Cepheid).
Image: The Xpert MTB/RIF assay loaded into the GeneXpert 4-module machine (Photo courtesy of Cepheid).
The diagnosis of smear-negative and extrapulmonary Tuberculosis (TB) remains challenging in resource-limited settings. Confirmation of smear-negative and extrapulmonary TB cases requires a positive Mycobacterium tuberculosis culture, which may take up to eight weeks.

The feasibility of the implementation of a commercial rapid molecular diagnostic test for the routine diagnosis of smear-negative or extrapulmonary tuberculosis (TB) and its diagnostic accuracy, and to assess HIV prevalence in a real-life setting has been evaluated.

A team of international scientists collaborating with those at the University Hospitals of Geneva (Geneva, Switzerland) conducted a prospective cohort study of all consecutive cases with suspected smear-negative and/or extrapulmonary TB over a two-year period. Cases were classified as proven, probable, or possible TB cases, or as having an alternative diagnosis. For suspected pulmonary TB patients, direct microscopic examination of sputum smears after Ziehl–Neelsen staining was performed at the local hospital in Madagascar.

All negative sputum acid-fast bacillus (AFB) smears from pulmonary TB patients and specimens from extrapulmonary TB patients were sent to the Mycobacteria Laboratory for a second smear examination, culture, and Xpert MTB/RIF screening. Standard analyses (cytological and biochemical) for biological fluids (cerebrospinal fluid, peritoneal fluid, pleural effusion) and the pathological examination of tissue samples (lymph nodes, cutaneous biopsies, pleural and synovial biopsies) were also performed. HIV serology testing was offered to all patients. HIV status was confirmed using three different conformational rapid tests.

The scientists found that of the 363 patients included, 183 (50.4%) had suspected smear-negative pulmonary TB and 180 (49.6%) had suspected extrapulmonary TB. For proven cases, the sensitivity, specificity, positive and negative predictive values of Xpert MTB/RIF were 82.4%, 98.8%, 98.3%, and 86.6%, respectively; for proven and probable cases grouped together, these values were 65%, 98.8%, 98.5%, and 64%, respectively. The diagnostic accuracy was slightly lower for extrapulmonary TB compared to smear-negative pulmonary TB. The prevalence of HIV infection was 12.1%, but almost half of these cases did not have TB (alternative diagnosis group).

The authors concluded that the implementation of a rapid diagnosis programme for TB in a resource-poor setting is feasible. The performance of the Xpert-MTB/RIF was remarkable in this difficult-to-diagnose population. HIV prevalence in this study was much higher than the prevalence reported in the general population in Madagascar, in patients with TB and patients with conditions other than TB. The study was published in the April 2018 issue of the International Journal of Infectious Diseases.

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University Hospitals of Geneva

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