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Rapid diagnosis of tuberculosis (TB) and antibiotic resistances are imperative to initiate effective treatment and to stop transmission of the disease. A new generation of more sensitive, automated molecular TB diagnostic tests has been recently launched giving microbiologists more choice between several assays with the potential to detect resistance markers for rifampicin and isoniazid. In this study, we determined analytical sensitivities as 95% limits of detection (LoD 95 ) for Xpert MTB/Rif Ultra (XP-Ultra) and BD-MAX MDR-TB (BD-MAX) as two representatives of the new test generation, in comparison to the conventional Fluoro-Type MTB (FT-MTB). Test matrices used were physiological saline solution, human and a mucin-based artificial sputum (MUCAS) each spiked with Mycobacterium tuberculosis in declining culture-and qPCR-controlled concentrations. With BD-MAX, XP-Ultra, and FT-MTB, we measured LoD 95 TB values of 2.1 cfu/ml (CI 95% : 0.9-23.3), 3.1 cfu/ml (CI 95% : 1.2-88.9), and 52.1 cfu/ml (CI 95% : 16.7-664.4) in human sputum; of 6.3 cfu/ml (CI 95% : 2.9-31.8), 1.5 cfu/ml (CI 95% : 0.7-5.0), and 30.4 cfu/ml (CI 95% : 17.4-60.7) in MUCAS; and of 2.3 cfu/ml (CI 95% : 1.1-12.0), 11.5 cfu/ml (CI 95% : 5.6-47.3), and 129.1 cfu/ml (CI 95% : 82.8-273.8) in saline solution, respectively. LoD 95 of resistance markers were 9 to 48 times higher compared to LoD 95 TB . BD-MAX and XP-Ultra have an equal and significantly increased analytical sensitivity compared to conventional tests. MUCAS resembled human sputum, while both yielded significantly different results than normal saline. MUCAS proved to be suitable for quality control of PCR assays for TB diagnostics.

BD MAX MDR-TB assay is a new molecular platform for the detection of Mycobacterium tuberculosis complex (MTBC) in clinical specimens and simultaneous detection of resistance toward isoniazid and rifampicin. This study assessed the assay's diagnostic accuracy by using pre-characterized MTBC cultureenegative (n Z 257), smear-negative/MTBC cultureepositive (n Z 93), and smear-positive/MTBC cultureepositive (n Z 153) respiratory specimens. Compared with culture, the overall sensitivity and specificity of BD MAX MDR-TB were 86.6% and 100%, respectively; sensitivities for smear-positive and smear-negative samples were 100% and 64.5%. Sensitivity and specificity for isoniazid and rifampicin resistance were 58.3% (biased low due to sample collection strategy in low prevalence setting), 99.3%, 100%, and 98.2%, compared with phenotypic drug resistance testing and 100%, 99.4%, 100%, and 99.4%, compared with GenoType MTBDRplus. In conclusion, BD MAX MDR-TB is an accurate assay for the diagnostic detection of MTBC in respiratory samples and its resistance toward the most important anti-TB drugs isoniazid and rifampicin. Due to its medium to high throughput, good validity, and ease of use, the assay will be of great benefit for medium-sized to large TB diagnostic centers.

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