Cephalothin is not a reliable surrogate marker for oral cephalosporins in susceptibility testing of Enterobacteriaceae causing urinary tract infection

- Vitek® 2 AST-N44 includes cephalothin as predictor for oral cephalosporins in uUTI.
- Vitek® 2 is not a reliable system for susceptibility testing of cephalothin.
- Cephalothin and cefazolin do not show a good correlation for the E. coli studied.
- Cefazolin shows a greater in vitro activity than cephalothin for the E. coli.
- Vitek® 2 AST-N44 should provide cefazolin instead of cephalothin as CLSI recommends.

Vitek® 2 (bioMérieux) is a widely used commercial antimicrobial susceptibility test (AST) system. AST-N244 card includes cephalothin as first-generation cephalosporin. We compared the cephalothin susceptibility results obtained with Vitek® 2 AST-N244 to those obtained by broth microdilution (BMD) and disk diffusion (DD) for 212 urinary Enterobacteriaceae. We also evaluated the differences between cefazolin and cephalothin susceptibility results. The overall performance of Vitek® 2 for cephalothin testing was 74.5% and 76.4% category agreement compared to BMD and DD, respectively; 84.4% essential agreement; very major errors 15.2% and 11.1% compared to BMD and DD; major errors 0% compared to both methods; and minor errors 22.2% and 21.7% compared to BMD and DD. Regarding correlation between cephalothin and cefazolin, the differences observed were statistically significant (P < 0.0001) for the 167 Escherichia coli included (39.5% cephalothin susceptible versus 92.2% cefazolin susceptible by BMD; 41.9% cephalothin susceptible versus 93.4% cefazolin susceptible by DD). Vitek® 2 should provide cefazolin instead of cephalothin as a surrogate marker for oral cephalosporins on the urinary AST-244 cards in order to follow the CLSI (2016) recommendations.