Evaluation of Genexpert Mtb/Rif Assay for Diagnosis of Pulmonary Tuberculosis and Detection of Rifampicin Resistance in Sputum Specimens At Kharamara Hospital, Jigjiga, Eastern Ethiopia
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Date
2014-06
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Addis Ababa University
Abstract
Background: TB detection rate remains critical stumbling block for TB control. Though
conventional methods for TB detection exist; GeneXpert MTB/RIF assay represents a paradigm
shift in the diagnosis of TB and MDR-TB by simultaneous detection of M.tuberculosis and
rifampicin resistance bacilli. However information regarding the performance characteristics of
Xpert MTB/RIF assay is scarce in Ethiopia.
Objective: To evaluate the performance of Xpert MTB/RIF assay for diagnosis of pulmonary
tuberculosis and detection of Rifampicin resistance in sputum specimens at Karamara hospital,
Jigjiga.
Materials and methods: A total of 227 paired expectorated sputum samples from patients with
signs and symptoms of suggestive of tuberculosis at Karamara hospital during February 2014 to
May 2014 were recruited by consecutive sampling method. Sputum sample was tested directly
by Ziehl-Neelsen staining and Xpert MTB/RIF assay without NALC-NaOH decontamination
and sputum samples were cultured for isolation and drug sensitivity testing of TB bacilli.
Diagnostic performance of Xpert MTB/RIF assay and AFB smear microscopy was calculated
using sensitivity, specificity, positive and negative predictive values, positive and negative
likelihood ratios against culture as the gold standard. Results with contaminated cultures (for
both LJ and MGIT) and failed Xpert MTB/RIF were excluded from analysis.
Results: Overall 25.5% (58/227) samples were positive for MTBC by MGIT and/or LJ media of
which 36.2 % (21/58) and 65.5% (35/58) were positive by AFB smear microscopy and Xpert
MTB/RIF respectively. The sensitivity, specificity, positive and negative predictive value of
Xpert MTB/RIF assay were 65.5% (95% CI: 53.3-77.7%), 96.3% (95% CI: 93.4-99.2%), 86.4%
(95% CI: 76.2-96.5%), and 88.6% (95% CI: 83.9-93.3%) respectively. Eighteen (31.0%, 18/58)
more cases, which were smear microscopy negative, were detected by Xpert MTB/RIF assay
Conclusion: Xpert MTB/RIF assay demonstrated high sensitivity in detecting MTBC in sputum
specimens compared with conventional AFB smear microscopy. However, it demonstrated
suboptimal sensitivity in smear negative patients compared to culture test.
Keywords: Xpert MTB/RIF assay, NPV, PPV, Sensitivity, Specificity
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Xpert MTB/RIF assay, NPV, PPV, Sensitivity, Specificity