Programme Hospitalier Recherche Clinique, Institut Pasteur, Inserm, French Public Health Agency.

The detection of extended-spectrum ␤-lactamase-producing (ESBL) bacteria is of importance for infection control and epidemiological surveillance. We aimed to compare phenotypic methods available in the routine laboratory and to evaluate two-step strategies using these methods for the detection of ESBL-positive Enterobacteriaceae. Two methods used for routine susceptibility testing (Vitek2 and disk diffusion methods) and seven methods designed for the detection of ESBL production (ESBL Etests, combination disks, double-disk synergy [DDS] methods on Mueller-Hinton [MH] agar and cloxacillin-containing MH agar, and the Cica-Beta test) were tested against 107 strains of Enterobacteriaceae not susceptible to extended-spectrum cephalosporins. All strains were screened for the presence of acquired ESBL-encoding genes by PCR, and the PCR result was considered the gold standard for evaluation of the other test methods. Among the 107 strains, 52 (49%) were ESBL positive. With Vitek2, sensitivities were the highest when using extended cards (73% to 79%), but 25% to 31% of the strains yielded indeterminate results. For the disk diffusion method, sensitivities were the highest (96%) when testing at least cefotaxime, cefepime, and a third compound (ceftazidime, cefpodoxime, or aztreonam). For the specific methods, specificities ranged from 62% (ceftazidime ESBL Etest) to 100% (DDS using a disk spacing of 20 mm). When a method designed for ESBL detection was used on strains considered ESBL negative or with an indeterminate result by a first routine susceptibility method, sensitivities reached 100% for a majority of combinations. In conclusion, two-step strategies using phenotypic methods available in most clinical laboratories may reach a sensitivity of 100% for ESBL detection among a large panel of species, including AmpC producers, providing a sensible choice of tests.