Diagnostic utility of auramine O smear microscopy for detecting non-tuberculous mycobacteria versus Mycobacterium tuberculosis complex in adult clinical samples: a 5-year retrospective study (2018-2022).

backgroundFluorochrome smear microscopy is the method recommended for the direct examination of clinical samples for mycobacteria. However, no studies to date have comprehensively assessed the diagnostic utility of this method using auramine O stain for detecting non-tuberculous mycobacteria (NTM) versus Mycobacterium tuberculosis complex (MTBC) in adult samples. Hence, this study aimed to investigate the diagnostic utility of auramine O smear microscopy (AOSM) for detecting NTM versus MTBC in adult samples, using mycobacterial culture as the reference standard.

methodsThis was a 5-year retrospective study conducted in a tertiary academic medical centre in Malaysia. Adult samples tested with both AOSM and mycobacterial culture from 1 January 2018 to 31 December 2022 were included in the analysis. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) with 95% confidence intervals (CI) were calculated using Open Epi Version 3.01.

resultsOf 12 016 adult samples that underwent both AOSM and mycobacterial culture, 19.0% (n = 2 288) were culture positive. NTM and MTBC accounted for 50.2% (n = 1 149) and 49.8% (n = 1 139) of the mycobacterial isolates, respectively. The diagnostic utility of AOSM for detecting NTM versus MTBC was 2.4% (95% CI 1.6%-3.5%) versus 53.6% (95% CI 50.6%-56.5%) for sensitivity, 93.8% (95% CI 93.3%-94.2%) versus 99.1% (95% CI 98.9%-99.3%) for specificity, 4.0% (95% CI 2.8%-5.8%) versus 86.4% (95% CI 83.8%-88.7%) for PPV and 90.1% (95% CI 90.0%-90.2%) versus 95.3% (95% CI 95.0%-95.6%) for NPV.

conclusionsThe diagnostic utility of AOSM was limited by poor sensitivity and PPV for NTM, and by poor sensitivity for MTBC. A positive AOSM result was a much more reliable indicator of the presence of MTBC than of NTM. In settings with a higher burden of TB than NTM disease, AOSM-positive patients with a clinical history suggestive of TB disease should be considered for TB treatment initiation while awaiting culture confirmation. In contrast, AOSM-positive patients whose clinical history is not strongly suggestive of TB-or is suggestive of NTM disease-should undergo additional testing using alternative methods, such as rapid molecular assays, that can reliably detect and distinguish between MTBC and NTM while awaiting culture confirmation.