Proposal for detection of extended-spectrum-β-lactamase with the Vitek ESBL-test

Objective To assess the validity of were used as Vitek antimicrobial susceptibility cards(VASC)for detection of ESBL and try to develop a nwe method to make the ESBL test more perfect.Methods First,268 strains of E.Coli and 208 strains of klebsiella spp.were detected for ESBL with Vitek CNS NT cards and the double disk diffusion method(DDM)respectively.The strains with contradictory results were retested with the agar dilution method(ADM)and two additional methods were used as references.Results Of the 476 tested strains,18 presented deferent VASC results from the didution method when they were detected with DDM and ADM.Out of the 18 strains,15 were negative with VASC method,but positive with DDM and ADM.Their characteristics are that,when clavulanate(CA)was added to ceftazidime(CAZ)and ceftaxime(CTX),the MIC of either drug was reduced by at least three twofold dilutions,but not to 0.5 μg/ml.Two strains showed ESBL negative with VASC and ADM and suspected to be positive with the DDM,and the other 1 was positive with the VASC,but negative with DDM and ADM.Testing with the methods for reference revealed that except the strain which was positive with the VASC,all the other 17 were both ESBL producing and AmpC producing strains.Conclusion Most ESBL producing strains can be detected with the VASC,but the strains which produce simultaneously other highly active enzymes,such as AmpC etc.,may give false ESBL negative.A remedy for this is proposed in this paper.When testing the strains which resist the first generation cephalosorins with the VASC but are ESBL negative,the Lab Report Window should be turned to the Raw Date Report Window for observing the A values of CTX,CTX/CA and CAZ,CAZ/CA.If the A values of either of the couple drugs are reduced by about 40% simultaneously,the ESBL should be tested with DDM again.ealedthatexceptthe