Abstract
Background: The cefazolin inoculum effect (CzIE), in which the minimum inhibitory concentration (MIC) of cefazolin increases depending on the bacterial inoculum, has been reported to affect the prognosis of infections caused by methicillin-susceptible Staphylococcus aureus (MSSA). However, evaluating CzIE using high inocula (HI) for all isolates is impractical in clinical settings. This study aimed to evaluate whether routine microbiological tests could help screen for CzIE in MSSA isolates. Methods: We analyzed 179 MSSA isolates detected from blood cultures at Chutoen General Medical Center between May 2013 and January 2023. CzIE was defined as a cefazolin MIC ≤ 8 μg/mL with standard inocula (SI) and ≥ 16 μg/mL with HI. Results: CzIE was observed in 9 isolates (5.0%). The frequency of CzIE was 0.0% (0/128) in isolates with SI MIC ≤ 0.5 μg/mL, 14.6% (7/48) with MIC = 1 μg/mL, and 66.7% (2/3) with MIC = 2 μg/mL. No CzIE was observed in β-lactamase-negative isolates. By excluding MSSA isolates with SI MIC ≤ 0.5 μg/mL or negative β-lactamase results, 128 isolates (72%) could be omitted from HI testing. Conclusion: The proposed exclusion criteria enabled efficient CzIE screening by reducing the number of additional tests without missing any CzIE-positive strains. Routine microbiological tests may serve as useful tools for evaluating CzIE in MSSA in clinical laboratories.
