A clinical microbiology lab receives a urine isolate of Klebsiella pneumoniae from a catheterized ICU patient. Disk diffusion shows resistance to ceftriaxone and ceftazidime but the organism tests susceptible to cefoxitin and imipenem. The phenotypic confirmatory test for ESBL production should be:

• A Nitrocefin chromogenic test detecting beta-lactamase hydrolysis of a chromogenic substrate
• B Modified Hodge test (MHT) with imipenem disk to detect carbapenemase production
• C Disk diffusion with aztreonam alone — inhibition zone <21mm confirms ESBL
• D Combined disk test — ceftazidime and cefotaxime disks alone vs. combined with clavulanate (≥5mm increase = ESBL positive) ✓

Explanation

The combined disk test (CDT) is the standard phenotypic confirmatory test for ESBL. It compares inhibition zones of ceftazidime and cefotaxime disks placed alone vs. in combination with clavulanate (4 μg). An increase of ≥5 mm in the zone diameter with the clavulanate-containing disk compared to the disk without clavulanate confirms ESBL production, because clavulanate is a class A beta-lactamase inhibitor that inhibits ESBLs (TEM, SHV, CTX-M type). Cefoxitin susceptibility and imipenem susceptibility help distinguish ESBL from AmpC-producing organisms and carbapenemases respectively. Nitrocefin tests detect all beta-lactamases, not ESBL specifically. MHT detects carbapenemases, not ESBLs.

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

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