Comparison of direct and standardized antibiotic susceptibility testing in bacteriuria

In clinical isolates of bacteria from urine, the antibiotic susceptibility was tested with a direct disc diffusion method in which the inoculum was not primarily standardized. The results were compared with those from parallel tests using standardized inoculum. Of a total 2,272 urine specimens, 700 were accepted for direct susceptibility testing, 189 were not accepted because of mixed culture or indistinct inhibition zones. The overall concordance between the direct and the standardized tests was 95.8% (2,800 observations with 118 discordant results). The direct diffusion method used in this way is time saving and economical and can reliably be used in a clinical microbiological laboratory serving mainly outpatients.