title: A comparison of agar dilution with the Calibrated Dichotomous Sensitivity (CDS) and Etest methods for determining the minimum inhibitory concentration of ceftriaxone against Neisseria gonorrhoeae.
abstract: OBJECTIVE
  The objective of this study was to compare the Calibrated Dichotomous Sensitivity (CDS) based agar dilution (CDS AD) method with the Etest (bioMérieux SA) methods using 2 method protocols for determining the minimum inhibitory concentration (MIC) of ceftriaxone against Neisseria gonorrhoeae. The two method protocols were the manufacturer's protocol for which the Clinical and Laboratory Standards Institute (CLSI) interpretative criteria for Neisseria gonorrhoeae could be applied, and the CDS-adapted protocol. Comparability of MIC data is critical for situation analysis and monitoring trends in global antimicrobial analysis.
  
  
  METHODS
  Two hundred and forty eight clinical isolates of N. gonorrhoeae and the World Health Organisation (WHO) N. gonorrhoeae reference strains were tested using the three methods.
  
  
  RESULTS
  When compared, CDS AD and CDS Etest gave a regression R(2) value of 94%, the Pearson's correlation coefficient was 97% and a paired comparison within one log2 dilution was 98%. The CDS AD and the Etest (CLSI) comparison gave a regression R(2) value of 90%, a Pearson's correlation coefficient of 95% and a paired comparison within one log2 dilution was 98%. The comparison of the CDS Etest and CLSI Etest gave a regression R(2) value of 91%, a Pearson's correlation coefficient of 95% and a paired comparison within one log2 dilution of 99%. Importantly, there was robust agreement between all three methods for the categorization of susceptibility of Neisseria gonorrhoeae isolates using the WHO nominated breakpoint for decreased susceptibility to ceftriaxone (≥0.125 μg/mL).
  
  
  CONCLUSIONS
  The CDS Etest method is comparable to agar dilution and the Etest methods for determining the MIC of ceftriaxone against N. gonorrhoeae.
authors.affiliation: WHO Collaborating Centre for Sexually Transmitted Diseases, Sydney, Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW, 2031, Australia; Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW 2031, Australia.
authors.forename: Rodney P
authors.initials: RP
authors.lastname: Enriquez
authors.affiliation: WHO Collaborating Centre for Sexually Transmitted Diseases, Sydney, Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW, 2031, Australia.
authors.forename: Namraj
authors.initials: N
authors.lastname: Goire
authors.affiliation: WHO Collaborating Centre for Sexually Transmitted Diseases, Sydney, Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW, 2031, Australia; Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW 2031, Australia.
authors.forename: Ratan
authors.initials: R
authors.lastname: Kundu
authors.affiliation: Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW 2031, Australia; School of Medical Sciences, Faculty of Medicine, The University of New South Wales, Kensington, NSW 2033, Australia.
authors.forename: Barrie J
authors.initials: BJ
authors.lastname: Gatus
authors.affiliation: WHO Collaborating Centre for Sexually Transmitted Diseases, Sydney, Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW, 2031, Australia; Department of Microbiology, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW 2031, Australia; School of Medical Sciences, Faculty of Medicine, The University of New South Wales, Kensington, NSW 2033, Australia. Electronic address: monica.lahra@health.nsw.gov.au.
authors.forename: Monica M
authors.initials: MM
authors.lastname: Lahra
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002443:Ceftriaxone / Q000494:pharmacology*; D006069:Gonorrhea / Q000382:microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D009344:Neisseria gonorrhoeae / Q000187:drug effects* / Q000302:isolation & purification
chemical_list: D000900:Anti-Bacterial Agents; D002443:Ceftriaxone
keywords: Agar dilution; CDS; CLSI; Ceftriaxone; Etest; MIC; Neisseria gonorrhoeae