Agar plate dilution method for routine antibiotic susceptibility testing in a hospital laboratory.

Abstract

The agar plate dilution method for routine antibiotic susceptibility testing has been used in this hospital laboratory since 1968. The experience has met with the approval of the laboratory staff, and the technic has provided clinicians with more precise and useful information than was formerly obtained with disk sensitivity methods. Two studies were performed to document the reliability of our method. Testing of 60 organisms by both agar dilution and broth dilution methods showed that inhibitory concentrations were identical or varied by only one dilution, with a single exception. To compare results obtained using antibiotic laboratory standards and commercial intravenous preparations, 120 strains were tested in duplicate. Minimal inhibitory concentrations with both forms of drugs were similar or varied by only one dilution in all cases. A “time-work” study showed that the agar dilution method can be performed economically in a routine laboratory setting. The agar dilution method is appropriate for use in hospital laboratories and has distinct advantages over the disk agar diffusion technic.