Comparison of direct and standard microtiter broth dilution susceptibility testing of blood culture isolates

Turbid broth (0.5 ml) from blood culture bottles was inoculated into 0.5 ml of brain heart infusion broth, incubated for 3 to 6 h, diluted 1:500 in distilled water, and then inoculated directly into microtiter broth dilution susceptibility trays to test for minimal inhibitory concentrations. The results were compared to the standard tests performed 24 h later on colonies from subculture plates. The minimal inhibitory concentrations measured by these two methods were compared in 1,875 organism-antibiotic tests. The two minimal inhibitory concentrations were identical in 86.0% and within one twofold dilution in 98.0% of the tests. An organism was judged to be susceptible by one method and resistant by the other in 13 tests (0.7%). These 13 discrepancies were distributed among several organism-antibiotic combinations; no more than two were seen for any one combination. Highly accurate susceptibility testing can be achieved by using direct inoculation of turbid blood culture broths.