Evaluation of the capabilities of disc diffusion and Vitek2-compact GN13 methods for testing antimicrobial susceptibility and screening of ESBLs Enterobacteriaceae clinical isolates

Objective To evaluate the capabilities of disc diffusion and Vitek2-compact GN13 methods for testing antimicrobial susceptibility of screening ESBLs (extended-spectrum β-lactamase) in Enterobacteriaceae clinical isolates. Methods A total of 93 Enterobacteriaceae strains were isolated from patients with intra-abdominal infections in 21 hospitals during 2011 to 2012. The in vitro minimum inhibition concentration (MIC) values of ampicillin-sulbactam, piperacillin-tazobactam, ertapenem, ceftazidime, ceftriaxone, cefepime, imipenem, amikacin, ciprofloxacin and levofloxacin were determined by disc diffusion, Vitek2-compact GN13 and broth microdilution methods, respectively. Categorical agreement (CA) rates of disc diffusion and Vitek2-compact GN13 methods were determined by using broth microdilution method as the reference method. The genes encoding ESBLs were screened in Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumonia), Klebsiella oxytoca (K. oxytoca) and Proteus mirabilis (P. mirabilis) strains by using PCR analysis and gene sequencing. Disc diffusion and Vitek2-compact GN13 methods were used for the phenotypic confirmatory test of ESBLs and the sensitivity, specificity, positive predictive value and negative predictive value of the two tests were evaluated. Results The CA values of disc diffusion and Vitek2-compact GN13 methods for the 10 antibiotics were all >90% as compared with broth microdilution method. The major error (ME) rate for ertapenem was 3.2% and the very major error (VME) rates for ampicillin-sulbactam, ceftazidime and cefepime tests were all 2.2% by using Vitek2-compact GN13 method. The sensitivity, specificity, positive predictive value and negative predictive value of disc diffusion and Vitek2-compact GN13 methods in the phenotypic confirmatory test of ESBLs were 96.7% (29/30), 100% (20/20), 100% (30/30) and 95% (19/20), respectively. Conclusion Both disc diffusion and Vitek2-compact GN13 methods could be used for testing the antimicrobial susceptibility and the detection of ESBLs in Enterobacteriaceae clinical isolates with the advantage of accuarcy. Attention should be paid to the posibillity of oaurance of ME and VME when testing ertapenem, ampicillin-sulbactam, ceftazidime and cefepime by using Vitek2-compact GN13 method. Key words: Disc diffusion method; Vitek2; Broth microdilution method; Categorical agreement; Extended spectrum β-lactamase