Table 4.

Laboratory tests for detection of extended spectrum β lactamases

Tests	Method and interpretation
Screening tests
Double disk approximation or double disk synergy	Disk of third generation cephalosporin placed at 30 mm distance from amoxicillin-clavulanic acid. Enhanced inhibition indicates ESBL
Combination disk	Uses two discs of third generation cephalosporin alone and combined with clavulanic acid. An increase in the zone inhibition of >5 mm with the combination disk indicates ESBL
Microdilution test	Growth in a broth containing 1 μg/ml third generation cephalosporin indicates ESBL
Confirmatory tests
MIC broth dilution	MIC of third generation cephalosporin alone or combined with clavulanic acid. A decrease in the MIC of the combination of ≥3 twofold dilutions indicates ESBL
E test (MIC ESBL strips)	Two sided strip containing ceftazidime on one side and ceftazidime-clavulanic acid on the other. The ratio of the MIC of the combination to that of ceftazidime alone of >8, or the presence of a phantom zone (or both) indicates ESBL
Automated instruments (for example, Vitek)	Measures MICs and compares growth of bacteria in presence of ceftazidime v ceftazidime-clavulanic acid
Molecular (DNA probes, PCR, RFLP)	Targets specific nucleotide sequences to detect different variants of TEM and SHV genes

ESBL=extended spectrum β lactamases; MIC=minimum inhibitory concentration; PCR=polymerase chain reaction; RFLP=restriction fragment length polymorphism.