Colorimetric Assays as the Diagnostic Modality of Pulmonary and Extra-pulmonary Tuberculosis in Resource-limited Settings

Abstract

Objectives: Diagnosis of tuberculosis (TB) is challenging, especially in a resource-limited setting. Non-commercial alternatives such as colorimetric assays must be used for effective diagnosis and prompt treatment. In this study we aimed to determine the diagnostic performance, time to detection, cost, contamination rate, and ease of performance of the oxidation-reduction assays; Resazurin tube assay (RETA) and Malachite Green decolorization assay (MGDA) and their modifications using para-nitro benzoic acid (PNB) for detection of TB against the results on Lowenstein-Jensen medium (LJ) medium. Methods: Two-hundred-seventeen samples were subjected to colorimetric assays by incubating the inoculated media at 370C. Dyes were added on days 10, 14, 18, 28, and 42 days of incubation, and color change was noted after 24 hours and was compared with results on the LJ medium. Results: Diagnostic performance of colorimetric assays increased from day 10 to day 28 (maximum at day 28), while it was the same on days 28 and 42. The sensitivity, specificity, and diagnostic accuracy of RETA were 87.6%, 95.7%, and 88.6%, while that of MGDA were 81.7%, 95.5%, and 86.5%, respectively. On day 18, more than 51% of the samples were positive by colorimetric assays, whereas only 20.4% were positive by LJ culture. In addition, the colorimetric assays were as economical as LJ culture. Conclusion: Colorimetric methods can potentially become non-commercial alternatives for rapidly detecting Mycobacterium. tuberculosis, especially in TB laboratories with limited resources. The ease of interpreting the results and its cost-effectiveness is an additional advantage. J Microbiol Infect Dis 2022; 12(3):108-115.

Keywords

• Mycobacterium tuberculosis
• colorimetric assays
• LJ culture
• malachite green
• resazurin

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