Evaluation of a Semiautomated Micro-broth Dilution System for Determining Minimum Inhibitory Concentrations of Antimicrobics

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Abstract
This study investigates the dispensing and inoculating characteristics of the M1C-2000* micro-broth dilution system and compares its ability to determine minimum inhibitory concentrations with that of the reference macro-broth dilution method. Microwell filling showed a coefficient of variation of 6.8%, and micro-well inoculating showed a coefficient of variation of 6.1%. The reference macro-broth dilution method showed a coefficient of variation of 2.9% for dispensing and a coefficient of variation of 2.9% for inoculating. Ninety-three and eight tenths per cent of macro-broth minimum inhibitory concentrations duplicated the macro-broth modal value, with 100% falling within ±double-dilution step. Eighty-five per cent of micro-broth minimum inhibitory concentrations duplicated the micro-broth modal value, with 99.8% falling within ±doubledilution step. When the micro-broth results were evaluated using the macro-broth modal minimum inhibitory concentration value as a reference point, 55.8% duplicated the macrobroth modal value, with 4.7% falling above it and the remainder being skewed below it, with 6.2% at -2 doubledilution steps. These findings indicate that minimum inhibitory concentrations measured by the MIC-2000 system are somewhat lower than those of the reference macro-broth method. As a result, the micro-broth minimum inhibitory concentrations show a wider dispersion when referenced to the macro-broth modal minimum inhibitory concentration than when referenced to their own modal value.

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