Incidence and clinical implication of trimethoprim/sulfamethoxazole susceptibility discrepancy between VITEK-2 and disc diffusion testing methods in Central Australia

Authors:

• Cramp, Georgia
• Naughton, William
• Mcleod, James
• Ullah, Shahid
• Tong, Steven Y.C.
• Tsai, Danny

Details:

Clinical Microbiology and Infection, 2025-12-31

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Objectives The objective of this study was to investigate the incidence and clinical implications of discrepancies between trimethoprim/sulfamethoxazole (TMP-SMX) susceptibility results produced by VITEK-2 and disc diffusion methods for Staphylococcus aureus. Methods A retrospective matched-cohort study was conducted in a remote Australian hospital on S. aureus clinical isolates with reported TMP-SMX resistance by VITEK-2 from 1 August 2020 to 31 August 2023. Patient demographics, clinical information, TMP-SMX susceptibility by disc diffusion, TMP-SMX prescriptions, 30-day mortality, and 30-day presentation to the hospital emergency department were collected. Susceptibility data were compared between two susceptibility testing methods—VITEK-2 and disc diffusion. Clinical outcomes of patients with skin-and-soft-tissue infections caused by S. aureus with discrepant susceptibility (TMP-SMX resistant by VITEK-2 but TMP-SMX susceptible by disc diffusion) who were prescribed TMP-SMX were compared with a control group (patients treated with TMP-SMX for TMP-SMX-VITEK-2 susceptible S. aureus) in an approximate 1:1 ratio. Results A total of 768 S. aureus isolates with reported TMP-SMX-resistance by VITEK-2 were identified and reassessed using disc diffusion. Only 13/768 (1.69%) were reported as TMP-SMX resistant by disc diffusion. A total of 168/755 patients with discrepant susceptibility received a course of TMP-SMX; 102/168 (60.7%) were women, and 154/168 (92.2%) were First Nations people. When comparing the discrepant-susceptibility group with the control, the number of 30-day emergency department presentations due to treatment failure of the original skin-and-soft-tissue infections treated with TMP-SMX was not significantly different (6/168 vs. 6/184, p 0.87). Conclusions We report a 98.3% (755/768) discrepancy rate of TMP-SMX resistance between VITEK-2 and disc diffusion methods in TMP-SMX-VITEK-2-resistant S. aureus isolates. There was no significant difference in outcomes between those with discrepant susceptibility and controls. This provides reassurance that TMP-SMX is effective in this setting and may significantly impact antimicrobial prescribing in settings with a high prevalence of methicillin-resistant S. aureus infections.