D-280. Evaluation of a New Etest Strip for AmpC Detection Using a Large Collection of Genotypically Characterized Strains
Session: Poster Session: β-Lactamase Detection in the Clinical Laboratory
Saturday, October 25, 2008: 12:00 AM
Room: Hall C
Simple and reliable methods for detection of plasmid mediated AmpC producers among clinical isolates of Enterobacteriaceae with elevated cefoxitin MIC results are needed for routine use in clinical laboratories. A new Etest AmpC strip based on cefoxitin (FX) ± cloxacillin (CLO) was compared to the available Etest AmpC strip based on cefotetan (CN)±CLO (ECCMID P874, 2008), albeit with limited numbers of test strains. An extended evaluation of Etest FX±CLO was performed using a large collection of globally diverse genotypically characterized strains producing plasmidic and chromosomal AmpC.
Etest FX (512-8 µg/mL) ±CLO double-sided strips were tested with 496 strains, 100 negative controls (60 ESBL producers) and 396 plasmidic or chromosomal AmpC strains from 10 countries included: E. coli (238), K. pneumoniae (111), K. oxytoca (8), P. mirabilis (1), C. freundii (19), S. marcescens (19), E. aerogenes (52), E. cloacae (33), Enterobacter spp (2), M. morganii (7), Salmonella spp (3), P. stuarti (2) and H. alvei (1). Genotypes (PCR based) included CMY-2, DHA-1, ACC, FOX and MIR. The Etest standard procedure for Gram negative aerobes was used and a reduction of cephamycin MIC by ≥3 dilutions by CLO or the appearance of a “phantom zone” or deformation of inhibition ellipses was interpreted as positive for AmpC. Etest off-scale results were interpreted as non determinable (ND).
Strains (N)Etest AmpC Results
Neg. controls10009820973
Detection of Plasmidic AmpC (%)
Sensitivity 8891
Specificity 9193

The new Etest AmpC strip based on FX±CLO showed acceptable sensitivity (91%) and specificity (93%) when challenged with a large collection of different plasmidic AmpC genotypes. This method could be used by clinical laboratories for routine detection and epidemiologic surveillance of AmpC producers.
Anette Engelhardt1, Anne Yusof2, Carolina Johansson2, Karin Sjöström1, Phion Ho2 and  A. Engelhardt,
AB BIODISK Role(s): Employee., (1)AB BIODISK, Solna, Sweden, (2)AB BIODISK