title: Spiral gradient endpoint susceptibility testing: a fresh look at a neglected technique.
abstract: OBJECTIVE
  Increasing antibiotic resistance and interest in matching antibiotic therapy with pharmacokinetic/pharmacodynamic characteristics of isolates has led to increasing demands for determination of MICs. This can lead to increased costs for the laboratory. The spiral gradient endpoint (SGE) technique, a low-cost method of MIC determination, was developed some years ago. Although the technique showed good correlation with reference methods, it was not widely employed, mainly due to the introduction of alternative methods. We have revisited this technique and evaluated it for the determination of MICs for fastidious organisms.
  
  
  METHODS
  The SGE method was first optimized for fastidious organisms using Haemophilus influenzae. Intra-batch and inter-batch reproducibility was determined for H. influenzae, Streptococcus pneumoniae, Moraxella catarrhalis and Neisseria gonorrhoeae. The method was then evaluated by comparison of MICs for clinical isolates of these organisms determined by SGE with those determined with the reference method.
  
  
  RESULTS
  Optimization of the technique resulted in a method with excellent reproducibility for all organisms tested [SD 0.10-0.337; coefficient of variation (CV) 8.59%-18.66%]. These SDs/CVs were lower than those of the reference methods (0.27-2.34; 31.0%-63.8%). There was excellent correlation of the MICs with the reference methods (0.908-0.930) and insignificant differences in numbers of strains in each resistance category, with no tendency for SGE to produce higher or lower MICs than the reference method (P > 0.05).
  
  
  CONCLUSIONS
  SGE was shown to be reproducible and produced results that correlated well with standard techniques for fastidious organisms. The method offers a rapid, flexible, cost-effective alternative for smaller laboratories and for routine use in developing countries.
authors.affiliation: Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR.
authors.forename: Rocky
authors.initials: R
authors.lastname: Pong
authors.forename: Maureen V
authors.initials: MV
authors.lastname: Boost
authors.forename: Margaret M
authors.initials: MM
authors.lastname: O'Donoghue
authors.forename: Peter C
authors.initials: PC
authors.lastname: Appelbaum
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects* / Q000302:isolation & purification; D001424:Bacterial Infections / Q000382:microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000191:economics / Q000379:methods*; D015203:Reproducibility of Results; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents