Spiral gradient endpoint susceptibility testing: a fresh look at a neglected technique

Objectives 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.