Fig 1.
Box-and-whisker plots showing the distribution of CT values with the chromosomal target of the B. anthracis real-time PCR assay (16) for polyester (A) and rayon (B) swabs spiked with virulent B. anthracis Ames strain cells at a concentration of 107 CFU/ml and then processed by the use of the three swab recovery methods. The bottom, middle, and top lines of each box correspond to the 25, 50, and 75% cumulative frequencies of the observed values, respectively. The endpoints of the whiskers show the 2.5 and 97.5 percentiles. For both swab materials, the differences in mean CT values between the three methods were found to be significant by one-way ANOVA (P < 0.001; n = 24). For polyester swabs, Tukey's multiple-comparison test revealed significant differences between all three methods (P < 0.001; n = 24). For rayon swabs, significant differences were identified between the SETS and the sonication method and the SETS and the vortex method (P < 0.001; n = 24); however, there was no significant difference between the sonication and vortex methods (P = 0.09; n = 24).