Inoculum Effect on Susceptibility Testing in Gram-Negative Pathogens

Antimicrobial therapy selection is driven by susceptibility testing results provided by the clinical microbiology laboratory to treating providers. Susceptibility testing guidance is provided by the Clinical and Laboratory Standards Institute (CLSI) in the U.S. and by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). When performing susceptibility testing, CLSI calls for a bacterial inoculum density of 500,000 ± 300,000 cfu/mL. However, differences in inoculum density may have an inoculum effect (IE) — that is, they may drastically alter minimum inhibitory concentration (MIC) values, particularly for beta-lactam drugs when beta-lactamases are being produced by the organism tested. In an era of gram-negative multidrug …