85 Detection of AmpC beta-lactamases in E. coli using different screening agars Table 1. Continued. Cefotaxime 1 mg/L Ceftazidime 1 mg/L Characteristics No. of isolates No Cefoxitin 8 mg/L Cefoxitin No Cefoxitin 8 mg/L Cefoxitin cAmpC positive and ESBL negative 40 40 (100%) 39 (97.5%) 40 (100%) 40 (100%) ESBL positive and AmpC negative 40 40 (100%) 17 (42.5%) 40 (100%) 15 (37.5%) blaCTX-M-1 group 28 28 (100%) 14 (50%) 28 (100%) 14 (50%) blaCTX-M-9 group 12 12 (100%) 3 (25%) 12 (100%) 1 (8.3%) AmpC negative and ESBL negative 39 1 (2.6%) 1(2.6%) 0 (0%) 0 (0%) Table 2 and 3 present the sensitivity and specificity values. Although the agars containing cefoxitin showed a slightly lower sensitivity then the agars without cefoxitin, all confidence intervals overlapped. Specificity was assessed for the 79 AmpC-negative isolates. The agars containing cefoxitin showed a better specificity than the agars without cefoxitin. The specificity of the cefotaxime 1mg/L + cefoxitin 8mg/L and ceftazidime 1mg/L + cefoxitin 8mg/L agar plates were similar, based upon overlap of confidence intervals. As described above, we found that ESBL isolates had the largest impact on specificity, since ~40% of them grew on the cefotaxime+cefoxitin or ceftazidime+cefoxitin agars. Table 2 Sensitivity (with 95% confidence intervals) of different agars Sensitivity of different agars* pAmpC cAmpC Cefotaxime 40/40 1.00 (0.92-1.00) 40/40 1.00 (0.92-1.00) Cefotaxime + Cefoxitin 39/40 0.98 (0.86-1.00) 39/40 0.98 (0.86-1.00) Ceftazidime 40/40 1.00 (0.92-1.00) 40/40 1.00 (0.92-1.00) Ceftazidime + Cefoxitin 39/40 0.98 (0.86-1.00) 40/40 1.00 (0.92-1.00) *Sensitivity based on growth on agar (true positive) divided through all isolates of group (true positive + false negative) 5
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