Diagnostic Discordance between Cefoxitin Screening by Vitek 2 Compact and Oxacillin Disc Diffusion in Staphylococcus aureus: A Cross-Sectional Study from Central India

Background: Automated antimicrobial susceptibility testing (AST) systems such as Vitek 2 are increasingly used in diagnostic microbiology. Cefoxitin screening via Vitek 2 serves as a surrogate for mecA-mediated methicillin resistance. However, discrepancies with conventional Oxacillin susceptibility testing remain a concern. Objectives: To assess the diagnostic concordance between Vitek 2 Cefoxitin screening and Oxacillin disc diffusion in Staphylococcus aureus isolates. Materials and Methods: This cross-sectional study analyzed 604 consecutive S. aureus isolates from diverse clinical samples tested at a tertiary care hospital in Central India (January–December 2024). Cefoxitin screening was performed using the Vitek 2 Compact (AST-GP 628 cards). Oxacillin susceptibility was determined by Kirby–Bauer disc diffusion (1 µg) on Mueller–Hinton agar with 2% NaCl as per CLSI M100 (2024). Diagnostic indices were calculated using Oxacillin results as comparator. Results: Among 604 isolates, 471 (78.0%) were Cefoxitin-positive and 133 (22.0%) Cefoxitin-negative by Vitek 2.Oxacillin disc diffusion identified 386 (63.9%) resistant and 218 (36.1%) sensitive isolates. CefoxitinVitek 2 screening demonstrated sensitivity 99.5%, specificity 60.1%, positive predictive value 81.5%, negative predictive value 98.5%, and overall accuracy 85.2%. Eighty-seven (18.5%) isolates were Cefoxitin-positive but Oxacillin-sensitive, while two (0.3%) were Cefoxitin-negative yet Oxacillin-resistant. Conclusion: Vitek 2 Cefoxitin screening is highly sensitive but moderately specific for MRSA detection. Discordant results underline the need for confirmatory mecA/PBP2a testing to prevent overestimation of MRSA prevalence.