Potential of two nucleic acid amplification assays for quantifying mycobacterial load in respiratory and non-respiratory specimens: a prospective study

Quantification of bacillary load plays a critical prognostic role in tuberculosis patients. This study evaluated the potential of the Cepheid GeneXpert (GX) MTB/RIF and the BD ProbeTec system as quantitative assays. The time to positivity (TTP) measured by the Mycobacterial Growth Index Tube system was compared to the cycle threshold (Ct) of GX MTB/RIF and the Metric Other than Acceleration (MOTA) scores generated by the ProbeTec system. Out of 714 samples examined, 44 culture confirmed cases were identified. The Ct values in 21 respiratory samples showed a high linear fit with the TTP in liquid culture (Spearman's correlation coefficient r = 0.9, P value < 0.0001), which was not the case in 23 non-respiratory samples. In both types of specimens, the MOTA scores did not correlate with the TTP in liquid culture. This indicates the suitability of GX as a quantitative measurement of mycobacterial load in respiratory but not non-respiratory specimens.