60 Chapter 4 Figure 3. Boxplots of model performance. The x-axis indicates the performance using Etests, AmpC Confirm Kit and MAST D68C. The y-axis indicates accuracy based on 10-fold three-times-repeated cross-validation using all 84 E. coli strains of the training set. ***P ≤ 0.001; ****P ≤ 0.0001; NS, not significant. Model description and performance The final RPART model contained two decisions and performance was evaluated on the training set (n=84). In the first decision—cefotaxime with an MIC of ≥6 mg/L for all pampC strains (n=42)—34 were correctly classified as pampC positive (n=34/42). In the second decision, samples with cefotaxime MIC <6 mg/L were divided by a cefotaxime MIC breakpoint of 0.50 mg/L. With an MIC breakpoint of cefotaxime <0.50 mg/L, 16 strains were correctly classified as negative (n=16/17). With a cefotaxime MIC ≥0.50 mg/L, all hyperproducer strains except one were correctly classified as hyperproducers (n=24/25). However, nine strains categorized as hyperproducers were either negative (n=1/17) or pampC positive (n=8/42) (Figure 4). This resulted in an overall model accuracy of 0.88 (95% CI=0.79–0.94) (Table 2). From an infection control perspective, it is most important to distinguish pampC from non-pampC. Therefore, we recalculated the performance from a three-class model to a two-class model. The negative and hyperproducer classes were merged to a non-pampC class. The twoclass model resulted in an accuracy of 0.90 (95% CI=0.82–0.96) with a sensitivity
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