Comparison of Double Disk Synergy Test and Combination Disk Test Methods for the Detection of Extended-Spectrum Beta-Lactamase Production among Enterobacteriaceae

Background: The emergence and spread of Extended-spectrum beta-lactamases (ESBLs) producing Enterobacteriaceae are serious and expanding public health problems globally. Rapid and accurate detection of these bacteria facilitates infection prevention efforts in healthcare facilities. The aim of this study was to determine the levels of agreement between double disk synergy test (DDST) and combination disk test (CDT) for the detection of ESBL among Enterobacteriaceae isolated from different clinical specimens. Methods: A total of 426 Enterobacteriaceae isolates were obtained from different clinical specimens from four clinical bacteriology laboratories. Fresh colonies of the isolates were recovered using MacConkey and 5% sheep blood agar plate. Antimicrobial susceptibility testing was performed using the Kirby-Bauer disk diffusion method on Muller Hinton agar (MHA). All the isolates were screened for ESBLs production using cefotaxime and ceftazidime as per CLSI guideline. Each ESBLs specious isolates were confirmed by CDT and DDST. The overall percent agreement, positive percent agreement (PPA) and negative percent agreement (NPA) was calculated. Data was entered and analyzed using SPSS version 20. Results: Using cefotaxime, 265 and ceftazidime, 259 were identified as potential ESBLs producing isolates. Of 265 screening ESBL positive Enterobacteriaceae, 92.8% (246/265) were confirmed as ESBLs producer by CDT, and 84.5% (224/265) by DDST. The most frequent Enterobacteriaceae were E. coli 228 (53.5%) and K. pneumoniae 103 (24.1%). The magnitude of ESBL was 246 (57.7%) by CDT and 224 (52.6%) by DDST. The highest frequencies of ESBL producers were found in blood specimen (84.4%), and from isolate in K. pneumoniae (85.4%). The overall percent agreement, positive percent agreement and negative percent agreement between DDST and CDT were 91.7%, 91.0 and 100%, respectively. Conclusion: The DDST provided a very good agreement with the CDT for the detection of ESBL. Since DDST method can be performed together with AST, it allows immediate, accurate clinical decisions to prescribe appropriate antibiotics treatment or isolation of patients. Detection of ESBL in conjunction with the routine antibiotics susceptibility can help for infection control caused by such bacteria and timely treatment of a patient with best antibiotics.