Evaluation of the chromogenic Cica-β-Test for detecting extended-spectrum, AmpC and metallo-β-lactamases

Background: Extended-spectrum, metallo- and AmpC β-lactamases usually are sought subsequently to susceptibility testing, meaning that producers are not identified until 72 h after a clinical specimen is taken. Chromogenic tests might usefully shorten this delay, and we investigated the Cica-β-Test for this purpose. Methods: Reference and clinical strains with known β-lactamases, or controls, were grown with a cefpodoxime disc to promote conservation of resistance. The cultures were then tested with nitrocefin and with the Cica-β-Test, which examines for hydrolysis of the chromogenic oxyimino-cephalosporin HMRZ-86 with and without specific inhibitors of extended-spectrum, metallo- and AmpC β-lactamases. Results were scored, as colour changes from yellow to red, with the tester blinded to the strain identity and the mechanism(s) present. Results: Proportions of extended-spectrum, metallo- and AmpC β-lactamase producers correctly identified by the Cica-β-Test were 85%, 77% and 72%, respectively. Such performance should be achievable if testing colonies from a primary culture plate, 24 h after a specimen was taken. Greater precision, albeit at more delay, would be achieved if results were read in conjunction with antibiogram data available 48 h after the specimen was taken. Limitations were frequent confusion of Klebsiella oxytoca hyperproducing K1 enzyme with AmpC hyperproducers, and that isolates with NMC-A or KPC carbapenemases were wrongly inferred to have AmpC enzymes. Conclusions: The Cica-β-Test has the potential to provide useful therapeutic guidance, identifying isolates with potent β-lactamases and informing early therapy; it will also help to monitor β-lactamase epidemiology among multiresistant strains.