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Extended-spectrum β-lactamase (ESBL) confirmation

Antibiotic Reference Laboratory

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.



Health Science

Lab name

Antibiotic Reference Laboratory

Lab location

Kenepuru Science Centre, Porirua

Test name

Extended-spectrum β-lactamase (ESBL) confirmation



Requested for surveillance


Notice required


Routine turnaround time

6 working days for reference specimens

Sample 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