Elevated Staphylococcus ceftriaxone MICs are an Etest artifact.

TO THE EDITOR— The recent publication by Pickering et al [1] described a collection of methicillin-susceptible Staphylococcus aureus (MSSA) that displayed elevated ceftriaxone minimum inhibitory concentrations (MICs) when tested by Etest (bioMerieux, Durham, North Carolina) gradient diffusion and would have been called “Resistant” to ceftriaxone based on previous Clinical and Laboratory Standards Institute (CLSI) interpretive guidance. The authors reported that approximately 60% of MSSA tested at their institution would have been misclassified based on the current CLSI guidance, which recommends testing staphylococci only against penicillin and oxacillin or cefoxitin in order to infer susceptibility or resistance to other β-lactam agents. This article was available electronically ahead of print for several months. Although it was subsequently retracted as “an honest error in interpretation,” we believe a fuller explanation of the findings could improve understanding among Clinical Infectious Diseases readership. We investigated the accuracy of the initial report by performing reference broth microdilution (BMD), disk diffusion, and Etest [both low (0.002–32 μg/mL) and high (0.016–256 μg/mL) range ceftriaxone Etest products] antimicrobial susceptibility testing on 8 pulsed field gel electrophoresis (PFGE)-matched pairs of MSSA from the Pickering study [1] reported to have mismatched ceftriaxone susceptibility. All 16 isolates were confirmed as oxacillin, cefoxitin, and ceftriaxone susceptible [2, 3] with BMD and diskmethods. CeftriaxoneMICs obtained by both Etest products were typically higher than those obtained with BMD but were still in the susceptible range for 100% of isolates using the high concentration ceftriaxone Etest, and for 93.8% of isolates using the low concentration ceftriaxone Etest (1 isolate tested as intermediate). In addition, 30 consecutive, unique MSSA isolated from blood cultures during 2 months at a single hospital were tested against ceftriaxone by BMD, disk diffusion, and Etest using a single 0.5 McFarland