Publication:

Phenotypic detection of presumptive AmpC β-lactamase production among cefoxitin screen-positive gram-negative bacilli in a tertiary care teaching hospital

Background AmpC β-lactamase-mediated resistance among Gram-negative bacilli is an increasingly important therapeutic challenge because it reduces the effectiveness of several β-lactam antibiotics and limits treatment options. In resource-limited laboratories, reliable phenotypic methods remain essential for the routine evaluation of cefoxitin screen-positive isolates for presumptive AmpC β-lactamase production.

Methods This prospective descriptive observational study was conducted at a tertiary care teaching hospital in Western Uttar Pradesh, India, from January to December 2025. A total of 2,860 clinical specimens yielded 1,486 clinically significant, non-duplicate Gram-negative isolates. Cefoxitin screen-positive isolates (inhibition zone diameter ≤18 mm) were evaluated using the Cefoxitin–Cloxacillin Double Disk Synergy Test (CC-DDST), Phenylboronic Acid Disc Test (PBA), and Disk Approximation Test (DAT) in accordance with CLSI M100, 35th edition (2025). Frequencies, percentages, and 95% confidence intervals were calculated.

Results Among the 1,486 Gram-negative isolates, 626 (42.1%; 95% CI: 39.6–44.6%) were cefoxitin screen-positive. Escherichia coli (40.6%) and Klebsiella pneumoniae (28.6%) were the predominant cefoxitin screen-positive isolates. CC-DDST identified the highest proportion of presumptive AmpC β-lactamase-positive isolates (303/626; 48.4%; 95% CI: 44.5–52.3%), followed by PBA (285/626; 45.5%; 95% CI: 41.6–49.4%) and DAT (206/626; 32.9%; 95% CI: 29.3–36.7%).

Conclusions A considerable proportion of Gram-negative isolates were cefoxitin screen-positive. Among the evaluated phenotypic methods, CC-DDST showed the highest positivity rate for presumptive AmpC β-lactamase production and appears to be a practical and cost-effective method for routine evaluation of cefoxitin screen-positive Gram-negative isolates in laboratories where molecular diagnostic facilities are unavailable.