Predictive Value of Direct Disk Diffusion Testing from Positive Blood Cultures in a Children's Hospital and Its Utility in Antimicrobial Stewardship

Abstract

Accurate and early susceptibility results could reduce overuse of broad-spectrum antibiotics for empirical treatment of bacteremia. Direct disk diffusion testing (dDD) using nonstandardized inocula directly from blood cultures could facilitate earlier narrowing of antibiotics. To determine the predictive value of dDD compared with standardized antimicrobial susceptibility testing (AST), we performed a retrospective cohort study of 582 blood cultures from 495 pediatric patients with bacteremia. Positive and negative predictive value (PPV: number of isolates susceptible by both dDD and AST divided by the total number of isolates susceptible by dDD; NPV: number of isolates not susceptible [either intermediate or resistant] by both dDD and AST divided by the total number of isolates not susceptible by dDD), sensitivity, specificity, and 95% confidence interval were calculated for each bacterium-antibiotic combination. We evaluated the Antibiotic Spectrum Index of prescribed antibiotics to assess change in antibiotic prescribing after availability of Gram stain, dDD, and AST results. dDD results were available a median of 21 h before AST results. dDD had PPVs of ≥96% for most organism-antibiotic pairs, including 100% (CI 96 to 100%) for Staphylococcus aureus with oxacillin and 99% (CI 93 to 100%) for Enterobacterales with ceftriaxone. NPVs of dDD were variable and frequently lower than the PPV. Very major errors and major errors occurred in 31/5,454 (0.6%) and 231/5,454 (4.2%) organism-antibiotic combinations, respectively. Antibiotics were narrowed in 30% of cases after a dDD result and a further 25% of cases after AST result. dDD is highly predictive of susceptibility for many common organism-antibiotic combinations and provides actionable information one day earlier than standard susceptibility approaches. dDD has the potential to facilitate earlier deescalation to narrow-spectrum antibiotic treatment.

Keywords: antimicrobial stewardship; antimicrobial susceptibility; bacteremia; diagnostic stewardship; pediatrics.