Validation of the VITEK2 and the advance expert system with a collection of enterobacteriaceae harboring extended spectrum or inhibitor resistant β-lactamases

Abstract

The susceptibility testing accuracy of the VITEK2 system and the ability of the Advance Expert System (AES) to provide interpretive readings were evaluated against 86 extended spectrum (ESBL) and 6 inhibitor-resistant-TEM (IRT) β-lactamases producing Enterobacteriaceae clinical isolates. VITEK2 MICs of 12 β-lactams were compared with those obtained by the standard NCCLS microdilution technique. The overall essential agreement ( ± 1 log dilution) was 87.8%. Discrepancies were mainly observed with cefepime (30.3% of total number of discrepancies), ceftazidime (21.2%), and cefotaxime (15.1%). MIC discrepancies were slightly higher in CTX-M- (14.4%) than in TEM- (12.5%) or SHV- (11.9%) type ESBL producers and were rare in IRT producers (1.4%). Overall interpretive agreement was 92.5% and minor, major, and very major errors were 5.4%, 1.7%, and 2.1%, respectively. The AES was able to identify an ESBL phenotype in 85 out of 86 isolates (98.8%) and an IRT phenotype in all 6 isolates harboring these enzymes, thus reducing very major errors to 0.9%. The VITEK2 system, in conjunction with the AES software, is a reliable tool for detection of ESBL or IRT producing Enterobacteriaceae isolates.