BACKGROUND: Because extended-spectrum -lactamase (ESBL) producing strains can frequent-ly cause therapeutic failure and infectious outbreaks in hospitals, rapid and accurate detection of these strains are important. We compared the Vitek ESBL test with the NCCLS ESBL phenotypic confirmatory test by disk diffusion (NCCLS ESBL test) and double disk synergy test (DDST). METHODS: For a total of 316 clinical isolates composed of Escherichia coli (184), Klebsiella pneu-moniae (120) and Klebsiella oxytoca (12), we performed the Vitek ESBL test and the NCCLS ESBL test. For sixty-eight ESBL producing isolates, the Vitek ESBL test was compared with the NCCLS ESBL test and the DDST. The ESBL producer was defined as an organism showing an increase in the inhibited zone diameter of >or=5 mm for either cefotaxime or ceftazidime in combination with clavu-lanic acid versus its single test. The DDST was performed with 20 mm and 30 mm for interdisk diam-eter. For seven false negative isolates in the Vitek ESBL test, the DDST of cefepime was performed. RESULTS: Compared with the NCCLS ESBL test, the Vitek ESBL test showed one false positive (specificity, 99.6%), seven false negatives (sensitivity, 89.7%) and 97.5% agreement. Seven false negative isolates of the Vitek ESBL test were the cefoxitin-resistant ESBL producer. In positivity for the NCCLS ESBL test of 68 ESBL producing isolates, cefotaxime-clavulanic acid and ceftazidime-clavulanic acid were 94% and 91%. Cefotaxime, ceftazidime, aztreonam and ceftriaxone showed 95/90%, 100/55%, 100/85% and 95/80% positivity in double-disk synergy with amoxicillin-clavulanic acid (AMC) for 20/30 mm of the interdisk diameter respectively. For seven false negative isolates of the Vitek ESBL test, cefepime showed a distinct synergic effect with AMC. CONCLUSIONS: The Vitek ESBL test may be a useful method for clinical laboratories due to its easy, rapid and sensitive method but its method was less sensitive to cefoxitin-resistant ESBL. For these cases, if the NCCLS ESBL test or DDST with cefepime are added, the detection rate of the ESBL pro-ducer can be augmented.