Comparison of Double Disc and Combined Disc Method for the detection of Extended Spectrum β Lactamases in Enterobacteriaceae

Objective: To compare double disc approximation and combined disc method for their ability to detect extended spectrum b lactamase (ESBL) production in enterobacteriaceae and determine the percentage of isolates which are falsely reported as sensitive in absence of ESBL detection, in a clinical microbiology laboratory of a tertiary care hospital between September - October 2002. Methods: Selected isolates were identified according to standard biochemical tests. Disc susceptibility tests were performed according to NCCLS. ESBL detection by combined disc {cefotaxime (30ug) versus cefotaxime plus clavulanate (30+10 ug)} was compared with detection using double discs {amoxy-clavulanic acid (20+10 ug) and aztreonam (30ug) applied 10 mm apart}. Results were interpreted according to NCCLS and analysed on SPSS version 10. Results: ESBL production was detected in 140 (30%) isolates by combined disc method and 139 (29.5%) by double disc method. There was no significant difference between two methods. Of the ESBL positive isolates 41(29%) gave zone diameters that were within the sensitivity range cutoff and would have been falsely reported as being beta lactam sensitive in absence of ESBL detection. Conclusion: ESBL detection should be routinely performed in clinical laboratories as false reporting would result in treatment failure despite in vitro sensitivity. No difference was found between the combined disc and double disc methods hence either of two could be used.