Results of an interlaboratory test on antimicrobial susceptibility testing of bacteria from animals by broth microdilution

A standard operating procedure for the determination of minimum inhibitory concentrations (MICs) of antimicrobial agents by the broth microdilution method was developed and evaluated for its fitness for use in an interlaboratory ring trial involving 46 routine diagnostic laboratories. All laboratories tested five strains (one reference strain and four field strains) against a total of 22 different antimicrobial agents. Gram-negative strains were tested against 16 different antimicrobial agents and Gram-positive strains against 14 different antimicrobial agents. Tests were performed once a week for three consecutive weeks. At least 80% of the results determined by 35 of the 46 participating laboratories were within the expected range (mode MIC ± 1 dilution step), with the 18 participating laboratories experienced in MIC determination showing a slightly higher mean percentage of accurate results (89.3% reproducible results) than the 28 non-experienced laboratories (86.7% reproducible results). The most accurate results were obtained for the Escherichia coli field strain, whilst the results for the Streptococcus uberis field strain showed the highest error rate. Among the 22 antimicrobial agents tested, the highest variabilities in the results (mean value for all antimicrobial agents 12.3%) were recorded for ceftiofur (27.8%), penicillin G (20.8%) and cefoperazone (20.6%).