Comparison of Broth Microdilution Versus MicroScan Walkaway Antimicrobial Susceptibility Testing in Predicting Enterococcus Faecalis Penicillin Susceptibilities
Faculty Mentor
Jaclyn Cusumano, Olivia Gladys Funk
Major/Area of Research
Pharmacy
Description
Background: Borderline-penicillin-resistant Enterococcus faecalis decreases the activity of first-line treatment ampicillin-ceftriaxone. As borderline-penicillin resistance rates exceed 25% in New York City, E. faecalis penicillin susceptibility must be accurately identified. This study compares the accuracy of rapid MicroScan Walkaway system to gold standard broth microdilution (BMD) in predicting E. faecalis penicillin susceptibility by minimum inhibitory concentration (MIC). Methods: A total of 227 E. faecalis clinical blood isolates across two medical centers (Albany, NY; New York, NY) were assessed for penicillin susceptibility using BMD, per CLSI standards, and MicroScan Walkaway. Isolates were classified as penicillin-resistant (MIC ≥16 μg/mL) or - susceptible (MIC ≤8 μg/mL) in accordance with CLSI. Susceptible isolates were further classified as borderline-resistant (MIC 4-8 μg/mL) or susceptible (MIC ≤2 μg/mL). Essential agreement was calculated by counting isolates with MICs identical to or one dilution from BMD values, while categorical agreement involved counting isolates matching BMD’s susceptibility classifications. Essential and categorical agreements were analyzed to identify very major errors (MicroScan susceptible, BMD resistant), major errors (MicroScan resistant, BMD susceptible), or minor errors (borderline-resistant by one method but not the other). Acceptable performance was defined as ≤1.5% very major, ≤3.0% major, or ≤10% minor errors. Conclusion: Based on BMD, 74.9% of isolates were susceptible, 25.1% were borderline-resistant and none were resistant. Based on MicroScan, 81% of isolates were susceptible, 16.7% were borderline-resistant and 0.9% were resistant. MicroScan more often underestimated MICs (20.7%) than overestimated (7.9%). Essential agreement was obtained in 97.4% of isolates. Per CLSI breakpoints, 99.1% of isolates had categorical agreement, with two major errors identified (0.9%). When considering borderline-resistant isolates, categorical agreement was 89.4%, with one major error (0.4%) and 23 minor errors (10.1%). Based on CLSI breakpoint, MicroScan displayed acceptable performance, but it does not demonstrate acceptable performance in predicting borderline-resistant penicillin MICs.
Comparison of Broth Microdilution Versus MicroScan Walkaway Antimicrobial Susceptibility Testing in Predicting Enterococcus Faecalis Penicillin Susceptibilities
Background: Borderline-penicillin-resistant Enterococcus faecalis decreases the activity of first-line treatment ampicillin-ceftriaxone. As borderline-penicillin resistance rates exceed 25% in New York City, E. faecalis penicillin susceptibility must be accurately identified. This study compares the accuracy of rapid MicroScan Walkaway system to gold standard broth microdilution (BMD) in predicting E. faecalis penicillin susceptibility by minimum inhibitory concentration (MIC). Methods: A total of 227 E. faecalis clinical blood isolates across two medical centers (Albany, NY; New York, NY) were assessed for penicillin susceptibility using BMD, per CLSI standards, and MicroScan Walkaway. Isolates were classified as penicillin-resistant (MIC ≥16 μg/mL) or - susceptible (MIC ≤8 μg/mL) in accordance with CLSI. Susceptible isolates were further classified as borderline-resistant (MIC 4-8 μg/mL) or susceptible (MIC ≤2 μg/mL). Essential agreement was calculated by counting isolates with MICs identical to or one dilution from BMD values, while categorical agreement involved counting isolates matching BMD’s susceptibility classifications. Essential and categorical agreements were analyzed to identify very major errors (MicroScan susceptible, BMD resistant), major errors (MicroScan resistant, BMD susceptible), or minor errors (borderline-resistant by one method but not the other). Acceptable performance was defined as ≤1.5% very major, ≤3.0% major, or ≤10% minor errors. Conclusion: Based on BMD, 74.9% of isolates were susceptible, 25.1% were borderline-resistant and none were resistant. Based on MicroScan, 81% of isolates were susceptible, 16.7% were borderline-resistant and 0.9% were resistant. MicroScan more often underestimated MICs (20.7%) than overestimated (7.9%). Essential agreement was obtained in 97.4% of isolates. Per CLSI breakpoints, 99.1% of isolates had categorical agreement, with two major errors identified (0.9%). When considering borderline-resistant isolates, categorical agreement was 89.4%, with one major error (0.4%) and 23 minor errors (10.1%). Based on CLSI breakpoint, MicroScan displayed acceptable performance, but it does not demonstrate acceptable performance in predicting borderline-resistant penicillin MICs.