Validation of a modified rapid test to detect the cefazolin inoculum effect in methicillin-susceptible Staphylococcus aureus from bloodstream infections in hospitals from North and Latin America

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BACKGROUND: The cefazolin inoculum effect (CzIE), defined here as a cefazolin MIC at high inoculum (107 colony-forming units/mL) ≥16 mg/L in MSSA, has been associated with less favourable clinical outcomes. However, detection of this phenotype is challenging in the clinical microbiology laboratory. We previously described modification of a rapid nitrocefin test using ampicillin disks rather than ampicillin powder for induction of the Staphylococcus aureus β-lactamase (BlaZ). OBJECTIVE: Evaluate the performance of the modified rapid nitrocefin test in a blinded fashion using MSSA isolates recovered from patients with bacteraemia. METHODS: We evaluated 200 MSSA isolates recovered from Latin American (LA) and North American (NA) hospitals (67 and 133 from NA and LA, respectively). The CzIE was determined using the modified rapid nitrocefin test with ampicillin disks and compared with MIC determination at high inoculum (gold standard). All isolates were subjected to whole-genome sequencing on an Illumina Hi-Seq platform. Performance metrics were calculated for the complete dataset and according to specific BlaZ types. RESULTS: The prevalence of the CzIE was 53% (105/200). Compared with the gold standard, the modified nitrocefin rapid test had a sensitivity of 96% and a specificity of 91.6%, with an overall accuracy of 94%. There were no false-positive results among blaZ-negative MSSA strains. CONCLUSIONS: The modified nitrocefin rapid test exhibited a robust performance to detect the CzIE in isolates from the Americas. This methodology is inexpensive and can be implemented in clinical microbiology laboratories around the world, including those with limited resources.

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