Suitability of Xpert MTB/RIF and genotype MTBDRplus for patient selection for a tuberculosis clinical trial

Abstract

Participation criteria for clinical trials in pulmonary tuberculosis commonly include confirmation of sputum positive for mycobacteria and an indication of drug susceptibility before treatment is initiated. We investigated the suitability of two novel sputum-based nucleic acid amplification methods for patient selection in a recent early bactericidal activity study. Spontaneously expectorated sputum samples of 140 consecutive pulmonary tuberculosis patients were examined with direct fluorescence microscopy, Genotype MTBDRplus assay (MTBDR), Xpert MTB/RIF assay (Xpert), and liquid mycobacterial culture. The methods detected mycobacteria or mycobacterial DNA in 96.8%, 90.5%, 92.9%, and 92.1% of samples, respectively. MTBDR, Xpert, and liquid culture were 100% concordant for detection of resistance to rifampin. Sensitivity and specificity of MTBDR for detection of isoniazid resistance were 83.3% and 100%, respectively. For quantification of mycobacterial sputum load, we found a correlation between Xpert DNA amplification cycle thresholds, time to positivity, and microscopy smear grade. The best correlation was found between Xpert and time to positivity (r = 0.54), which were both correlated with smear microscopy with r values equal to -0.40 and -0.48, respectively. We conclude that MTBDR and Xpert are suitable screening tools for determining rifampin resistance in sputum microscopy smear-positive patients before participation in tuberculosis trials. Xpert should be further explored as a surrogate measurement for sputum mycobacterial load.

Fig. 1.

One hundred forty patients provided a sputum sample for a direct smear. Detection of mycobacteria with all tests available could be directly compared between 126 patients (Box A). A quantitative comparison between Xpert and TTP was possible on 74 patients with a valid TTP result (Box B). Drug resistance detection (Box C) could be compared between NAATs in 118 patients, and the NAATs could be compared to MGIT-based DST in 95 patients.

Fig. 2.

Linear correlation between Xpert cycle thresholds and MGIT TTP. The correlation is moderate but significant (P < 0.0001), with r equal to 0.5394 (Pearson).