Objective: To detect the prevalence rate of the plasmid-mediated AmpC expressed constitutively at high level in Escherichia coli and Klebsiella pneumoniae for providing guideline for clinical therapy. Methods: Cefoxitin susceptibility disk test (K-B method) was used for screening and indirect cefoxitin three-dimensional test was used for the confirming of plasmid-mediated AmpC. Results: Of 86 strains

9 isolates were positive by K-B screening method. Of the 9 positive isolates

5 (three Escherichia coli

two Klebsiella pneumoniae) were positive by indirect cefoxitin three-dimensional confirming test

with a positive rate of 6.9%. Conclusion:Cefoxitin screening test combinated with indirect three-dimensional test can detect plasmid-mediated AmpC accurately and can be used in clinical routine detection.