DIAGNOSTIC TEST OF SPUTUM GENEXPERT MTB/RIF FOR SMEAR NEGATIVE PULMONARY TUBERCULOSIS

Abstract

The objective of this study was to evaluate the performance of the Gene-Xpert MTB/RIF sputum test for diagnosing pulmonary tuberculosis (TB) among patients sputum acid-fast bacillus (AFB) smear negative results in Thailand, a country with a high prevalence of pulmonary tuberculosis. We studied 151 patients who presented to Srinagarind Hospital, Khon Kaen, Thailand with a 2 week or more history of fever and/or cough and an abnormal chest radiograph between 2010 and 2014; these patients had at least 2 negative sputum AFB smear results. Of these, 76 were diagnosed as having either confirmed or probable pulmonary TB: the 32 confirmed cases were those with a positive sputum culture for Mycobacterium tuberculosis (MTB) and the 44 probable case were those with clinical and radiographic findings consistent with TB and who had a response to anti-TB therapy. Seventy-five cases were diagnosed as not having pulmonary TB. Of the 32 patients with a positive sputum culture for MTB, 26 had a positive GeneXpert MTB/RIF sputum test. Compared to sputum culture for MTB the GeneXpert MTB/ RIF test gave a sensitivity of 83.9% (95% CI: 66.3-94.5) and a specificity of 92.1% (95% CI: 83.6-97), a positive predictive value (PPV) of 81.3% (95% CI: 63.6-92.8) and a negative predictive value (NPV) of 93.3% (95% CI: 85.1-97.8). The GeneXpert MTB/RIF test had a fair sensitivity and specificity for diagnosing smear negative pulmonary TB. It may be useful for diagnosing pulmonary TB in patients with a negative sputum AFB smear. The assay is faster than culture and can detect rifampicin resistant strains of MTB.