Detection of extended-spectrum b-lactamases in members of the family Enterobacteriaceae: comparison of the double-disk and three-dimensional tests

A three-phase analysis of the Vitek ESBL test and a double-disk (2-disk) test was performed to assess their ability to detect extended-spectrum b-lactamases (ESBLs) in members of the family Enterobacteriaceae. In the first two phases involving detection of ESBLs in 157 strains possessing well-characterized b-lactamases, sensitivity and specificity were found to be 99.5 and 100%, respectively, for the Vitek ESBL test and 98.1 and 99.4%, respectively, for the 2-disk test. In the third phase, in which the ability of each test to detect ESBLs in 295 clinical isolates was assessed, there was only one false positive (Vitek ESBL test). Across all three phases, the Vitek ESBL test was found to be much easier to perform than the 2-disk test. The latter also involved subjective interpretation of results. There were a total of 176 Escherichia coli and 157 Klebsiella pneumoniae isolates and less than 40 isolates of each of 14 other species evaluated. In a supplemental study of Klebsiella oxytoca, an organism possessing a chromosomal b-lactamase similar to an ESBL, the Vitek ESBL test was found to be capable of detecting hyperproduction of this enzyme in strains of this species as well. These data indicate that the Vitek ESBL test is reliable for the detection of ESBLs in E. coli and K. pneumoniae, the two species in which ESBLs are most common, and of hyperproduction of the K. oxytoca b-lactamase, a situation which engenders a level of resistance to this species similar to that seen with ESBLs. After the widespread use of the expanded-spectrum cepha