Comparison of lower limits of detection of the GeneXpert MTB/RIF assay in seeded samples. Clinical specimens were spiked with dilutions of single-celled suspensions of M. tuberculosis. The lower limit of detection of the GeneXpert MTB/RIF assay in CSF (n = 9), gastric aspirate (n = 13), tissue (n = 8), and stool (n = 12) samples was compared to that in saline solution (n = 13). Wilcoxon's test was used to compare differences in medians. The asterisks indicate statistically significant difference. *, P ≤ 0.05; ***, P ≤ 0.0005; ns, not significant.