title: Utility of direct susceptibility testing on blood cultures: is it still worthwhile?
abstract: Earlier targeted therapy for bacteraemia optimizes patient outcomes and reduces broad spectrum antibiotic use. Standardized susceptibility testing results are available at 36-48 h. Direct disc susceptibility testing from blood culture broth reduces time to results but the inoculum is not standardized. No studies have looked at the clinical utility of direct susceptibility results. This retrospective cohort study aimed to assess the correlation between direct and formal testing methods as well as the clinical utility of direct susceptibility results. 160 episodes of bacteraemia with paired direct and formal susceptibility testing were studied. Direct disc testing was performed on blood culture broth. Formal testing was performed on isolates, using automated broth microdilution or Etests. The rate of error was 9.0 % (95 % CI 7.0-11.6 %). In 10 cases (6.3 %, 95 % CI 3.0-11.2 %), inappropriate antibiotics were used due to direct susceptibility results, including two cases with ineffective (as opposed to too broad) antibiotics being used. Antibiotics were changed in 28.1 % of cases once direct susceptibility data was available. There was a decreased time to effective antibiotics in 9.3 % (95 % CI 5.3-15.0 %), and a decreased time to a targeted antibiotics in 14.3 % (95 % CI 9.3-20.8 %) of cases. Despite the error rate, the advantages of earlier times to effective and targeted antibiotics justifies continuing direct testing in bacteraemia episodes with Gram-negative rods. In the Gram-positive group, given the contamination rate, the availability of adjunctive PCR, and the fact that early identification of the isolate could equally influence antibiotic choices, direct susceptibility testing may no longer be warranted.
authors.affiliation: Department of Microbiology and Infectious Diseases, Royal Prince Alfred Hospital, Camperdown NSW 2050, Australia.
authors.forename: Vidthiya
authors.initials: V
authors.lastname: Menon
authors.affiliation: Department of Microbiology and Infectious Diseases, Royal Prince Alfred Hospital, Camperdown NSW 2050, Australia.
authors.forename: Sophie
authors.initials: S
authors.lastname: Lahanas
authors.affiliation: Department of Microbiology and Infectious Diseases, Royal Prince Alfred Hospital, Camperdown NSW 2050, Australia.
authors.forename: Catherine
authors.initials: C
authors.lastname: Janto
authors.affiliation: Department of Microbiology and Infectious Diseases, Royal Prince Alfred Hospital, Camperdown NSW 2050, Australia.
authors.forename: Andie
authors.initials: A
authors.lastname: Lee
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016470:Bacteremia / Q000382:microbiology*; D001424:Bacterial Infections / Q000382:microbiology; D015331:Cohort Studies; D024881:Drug Resistance, Bacterial; D006801:Humans; D016133:Polymerase Chain Reaction / Q000379:methods*; D012189:Retrospective Studies; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D000900:Anti-Bacterial Agents