COMPARISON OF SPIRAL GRADIENT END-POINT AND AGAR DILUTION METHODS FORSUSCEPTIBILITY TESTING OF ANAEROBIC-BACTERIA - A MULTILABORATORY COLLABORATIVE EVALUATION

A multilaboratory collaborative study was carried out to assess the ut ility of the spiral gradient endpoint (SGE) method for the determinati on of the antimicrobial susceptibilities of anaerobes and to evaluate the equivalence of the MICs obtained by the SGE method with those obta ined by the reference agar dilution method of the National Committee f or Clinical Laboratory Standards. The standard deviation of the MIC ob tained by the SGE method for the five participating laboratories was /-0.26 of a twofold dilution, whereas it was +/-1 twofold dilution by the reference method. The interlaboratory reproducibility of the resul ts for two control strains tested with imipenem, chloramphenicol, and metronidazole indicated that 96% of the measurements fell within +/-1 twofold dilution of the mode. The equivalence of the SGE method with t he agar dilution method was assessed with a wide variety of anaerobic organisms. The MICs by both methods were within 1 doubling dilution in 93% of the measurements (n = 1,074). Discrepancies generally occurred with those organism-drug combinations that resulted in tailing endpoi nts (Fusobacterium nucleatum, 86% agreement) or in cases of light grow th (Peptostreptococcus spp., 86% agreement).