Extended-spectrum β-lactamase (ESBL) confirmation

Registry Details

Area Health Science
Lab name Antibiotic Reference Laboratory
Lab location Kenepuru Science Centre, Porirua
Test name Extended-spectrum β-lactamase (ESBL) confirmation
Organisms Enterobacterales
Requested for surveillance no
Notice required no
Routine turnaround time 6 working days for reference specimens
Culture requirements Pure culture
Media requirements A slope of any medium that supports the growth of the organism
Pre-transport requirements Ambient temperature
Transport requirements Ambient temperature
Unacceptable sample types Clinical specimens will not be accepted.
How results reported The results will state whether ESBL production was confirmed. In addition, the results of the CLSI disc confirmatory test and the double -disc synergy test (if done) will be reported.
Referral link Form


ESBLs are plasmid-mediated β-lactamases that confer resistance to third- and fourth-generation cephalosporins and the monobactams. They are inhibited by β-lactamase inhibitors, such as clavulanic acid. ESBLs are most common in Escherichia coli and Klebsiella species, but can occur in other Enterobacterales, such as Enterobacter.

Routinely, the CLSI disc confirmatory test is used to confirm ESBL production in E. coli, K. pneumoniae, K. oxytoca and Proteus mirabilis. If in the CLSI test, K. oxytoca isolates appear to have an ESBL, but are ceftazidime susceptible, they are further tested to distinguish between ESBL production and hyperproduction of the chromosomally mediated OXY β-lactamase.

Enterobacterales other than E. coli, K. pneumoniae, K. oxytoca and P. mirabilis are tested by both the CLSI disc confirmatory test and the double-disc synergy (Jarlier) test, using fourth-generation cephalosporins as well as third-generation cephalosporins to assist in the identification of ESBLs in the presence of AmpC β-lactamase.