Comparative Evaluation of Conventional and Automated Antimicrobial Susceptibility Testing Methods in a Tertiary Hospital Laboratory in Enugu State

Accurate and timely antimicrobial susceptibility testing (AST) is essential for guiding rational antibiotic therapy. The transition from conventional Kirby-Bauer disk diffusion to automated AST systems has the potential to improve laboratory efficiency and data quality in tertiary hospital settings. This study compared the diagnostic performance of conventional disk diffusion, broth microdilution, and the VITEK 2 automated AST system using clinical bacterial isolates at a tertiary hospital laboratory in Enugu State, South East Nigeria. One hundred and fifty clinical isolates comprising E. coli, K. pneumoniae, S. aureus, and P. aeruginosa were tested concurrently by all three methods for 10 antibiotics. Categorical agreement, essential agreement, major error rate, and very major error rate were calculated using broth microdilution as the reference standard, per EUCAST criteria. Results showed that VITEK 2 achieved 94.1 percent categorical agreement with broth microdilution, compared to 88.6 percent for disk diffusion. Major error rates for disk diffusion (4.2%) exceeded the acceptable 3 percent threshold for two antibiotic-organism combinations. VITEK 2 provided results 18 hours faster on average than conventional methods. Cost analysis revealed that VITEK 2 operational costs were 3.2-fold higher per isolate than disk diffusion. These findings support the diagnostic superiority of automated AST while contextualizing its economic challenges in resource-limited settings and recommending a hybrid approach combining automated and manual methods. Keywords: antimicrobial susceptibility testing, VITEK 2, disk diffusion, categorical agreement, Enugu.