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

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Abstract
Turbid broth (0.5 ml) from blood (human) culture bottles was inoculated into 0.5 ml of brain heart infusion broth, incubated for 3-6 h, diluted 1:500 in distilled water and then inoculated directly into microtiter broth dilution susceptibility trays to test for minimal inhibitory concentrations [MIC]. The results were compared to the standard tests performed 24 h later on colonies from subculture plants. The MIC measured by these 2 methods were compared in 1875 organism(bacteria)-antibiotic tests. The 2 MIC were identical in 86.0% and within one 2-fold dilution in 98.0% of the tests. An organism was judged to be susceptible by 1 method and resistant by the other in 13 tests (0.7%). These 13 discrepancies were distributed among several organism-antibiotic combinations; no more than 2 were seen for any 1 combination. Highly accurate susceptibility testing can be achieved by using direct inoculation of turbid blood culture broths.