Predictive value of direct disk diffusion testing from positive blood cultures in a children's hospital and its utility in antimicrobial stewardship.

Savage, Timothy J; Rao, Shun; Joerger, Jill; Ozonoff, Al; McAdam, Alexander J; Sandora, Thomas J

BackgroundAccurate and early susceptibility results could reduce overuse of broad-spectrum antibiotics for empiric treatment of bacteremia. Direct disk diffusion testing (dDD) using non-standardized inocula directly from blood cultures could facilitate earlier narrowing of antibiotics.MethodsTo 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.ResultsdDD results were available a median of 21 hours before AST results. dDD had PPVs of ≥96% for most organism-antibiotic pairs, including 100% (CI 96-100%) for Staphylococcus aureus and oxacillin and 99% (CI 93%-100%) for Enterobacterales and ceftriaxone. NPVs of dDD were variable and frequently lower than PPV. Very major errors and major errors occurred in 31/5454 (0.6%) and 231/5454 (4.2%) organism-antibiotic combinations, respectively. Antibiotics were narrowed in 30% of cases after dDD result and a further 25% of cases after AST result.ConclusionsdDD 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 de-escalation to narrow-spectrum antibiotic treatment.