In order to further challenge the performance of the 4 extraction systems to exclude inhibitors of rRT-PCR, 92 stool samples with levels of inhibition ranging from a single cycle to complete inhibition were evaluated. These samples were selected from among archived stool samples previously tested for enterovirus and MS2 after extraction by QIAamp Viral RNA Mini Kit. A sufficient number of samples with high, intermediate, and low levels of inhibitors were chosen for re-analysis to enable comparison between extraction procedures at each of these levels of inhibition. The results of MS2 inhibition for each sample by extraction protocol is shown in Fig. 1 and summarized by levels of inhibition in Table 1 , Experiment 2. The number of samples with inhibitors differed among the four protocols and for some individual samples, the amount of inhibition varied depending on the extraction protocol. Specifically, inhibitors were present in 58 (63.0%), 55 (59.8%), 37 (40.2%) and 30 (32.6%) of the samples prepared by Protocols A, B, C, and D, respectively. Median IQR reduction in Ct levels for protocols A, B, C and D were 11.7 (0 to 34), 7.0 (0 to 30), 4.2 (0 to 29), and 1.3 (0 to 30), respectively. More importantly the number and proportion of samples where inhibition was .6 Ct also varied among the protocols; 44 (47.8%), 29 (31.5%), 14 (15.2%), and 1 (1.1%), for Protocols A, B, C, and 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.