Performance of two combination disk methods as confirmation for ESBL and AmpC presence in clinical Enterobacterales isolates

Purpose: Guidelines recommend the use of a combination disk method (CDM) including cefepime and clavulanic acid as a confirmation method for ESBL detection. However, an alternative CDM containing cloxacillin (combining ceftazidime and cefotaxime with clavulanic acid and/or cloxacillin), is not only able to confirm ESBL presence, but also to provide information about AmpC production. We aim to show non-inferiority of the cloxacillin-CDM compared to the cefepime-CDM. Methods: We collected 102 individual clinical Enterobacterales isolates that were positive in the ESBL screening with the VITEK II. Phenotypic confirmation was performed with a cefepime-CDM and a cloxacillin-CDM (Rosco®). These results were compared with the results of a multiplex ESBL real-time PCR and an in-house developed conventional AmpC PCR. Results: Twenty-eight% of the isolates were positive in the ESBL PCR (CTX-M, SHV-2, TEM-3). The results of cefepime-CDM and cloxacillin-CDM were concordant with the ESBL PCR in respectively 99 and 94 %. The cefepime-CDM had a sensitivity of 100 % (95 % confidence interval (CI) 88,1-100) and specificity of 98,6 % (95 % CI 92,6-100). The cloxacillin-CDM had a sensitivity of 96,6 (95 % CI 82,2-99,9) and a specificity of 93,2 % (95 % CI 84,7-97,7). The cloxacillin-CDM in the group I Enterobacterales compared to the AmpC PCR had a sensitivity of 92 % (95 % CI 62-100) and a specificity of 100 % (95 % CI 77-100). Conclusion: Both cefepime-CDM and cloxacillin-CDM performed well as ESBL confirmation methods. Therefore, we suggest to add the cloxacillin-CDM as an alternative ESBL confirmation method to AMR detection guidelines in clinical settings.