Ninety-three stool suspensions were extracted by the four extraction systems after spiking the lysis buffer of each with MS2 that yielded similar final MS2 concentrations. Any contribution from endogenous MS2 RNA was ruled out since no MS2 RNA was amplified from any of these 93 stool suspensions upon extraction with protocol D when exogenous MS2 was omitted from the lysis buffer. Results for each extraction procedure by assigned levels of inhibition are shown in Table 1 . Individual values are shown in Fig. S1 . The proportion of samples with inhibitors varied among the extraction methods. Of the 93 RNA extracts, 12 (13.0%), 26 (28.3%), 7 (7.6%), and 7 (7.6%) extracted by protocols A, B, C, and D, respectively, contained inhibitors of MS2 rRT-PCR. Median interquartile range (IQR) reduction in Cts for protocols A, B, C and D were 2.4 (0 to 34), 1.9 (0 to 33), 0.58 (0 to 29), and 0.26 (0 to 7), respectively. Finally, the number of samples with an inhibition .6 Ct were 6 (6.5%), 3 (6.4%), 2 (2.2%), and 1 (1.1%) for Extraction Protocols A through D, respectively. The Friedman Test indicated that there was a statistically significant difference in co-extraction of inhibitors of MS2 rRT-PCR among the extraction protocols used, P,0.001.