For detecting ESBL-producing E. coli in a clinical isolate, the CLSI-recommended phenotypic confirmatory test is:

• A Positive mCIM combined with negative eCIM (EDTA-modified CIM)
• B EDTA-disc synergy test — zone enhancement indicates zinc-dependent enzyme
• C Carbapenem MIC >2 µg/mL by broth microdilution
• D Combined disc test: cefotaxime and ceftazidime alone vs. with clavulanic acid — ≥5 mm enhancement confirms ESBL ✓

Explanation

CLSI phenotypic confirmatory test for ESBL uses combination discs: cefotaxime and ceftazidime each with and without clavulanic acid (4 µg/mL). A ≥5 mm increase in zone diameter with clavulanic acid compared to without confirms ESBL production. This works because clavulanic acid inhibits ESBLs (class A beta-lactamases). mCIM/eCIM is used for carbapenemase detection, not ESBL. EDTA synergy detects MBL. ESBLs rarely cause carbapenem resistance alone.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.