Comparative Analysis of Two Commercial Automated Systems with Agar Dilution for Oxacillin Susceptibility and Their Association with Genotypes of Invasive Isolates (2011-2021).

Background: Determining oxacillin susceptibility using reference methods and automated systems is crucial for treating invasive infections caused by . This study compares the oxacillin susceptibility results from the two automated systems with agar dilution and correlates them with genotypes of invasive .

Methods: Non-duplicate invasive isolates were collected over an 11-year period. The oxacillin susceptibility was determined with Phoenix 100 (Jan 2011 to Aug 2018) or Vitek 2 (Sep 2018 to Dec 2021), and susceptibility for oxacillin and cefoxitin was determined with agar dilution. Methicillin-resistant (MRSA) was confirmed with existence, and the genotype was determined using SCC. The association between genotype and antibiotic susceptibility using two automated systems and agar dilution was evaluated.

Results: A total of 842 invasive , including 443 + MRSA and 399 - MSSA, were collected. The susceptibility rates of oxacillin determined by two automated systems and agar dilution were 68.8% (76.8% for Phoenix 100 and 57.6% for Vitek 2) and 54.0%, respectively. When compared with the oxacillin susceptibility using agar dilution, the categorical agreement for Phoenix 100 and Vitek 2 were 0.46% and 0.88%, respectively ( < 0.001). One hundred and forty-three isolates were misinterpreted as oxacillin-susceptible (OSSA) using automated systems while comparing with agar dilution, among which molecularly community-associated MRSA (CA-MRSA) outnumbered healthcare-associated MRSA (HA-MRSA) (99 vs 34, < 0.001). There were 70 + OSSA (OS-MRSA) using agar dilution, among which 42 harbored SCC types were predominantly categorized as CA-MRSA (38, < 0.001).

Conclusion: The categorical agreement of Vitek 2 in determining oxacillin susceptibility and predicting existence is comparable with agar dilution, whereas Phoenix 100 is not. Most of those ORSA determined by agar dilution but misinterpreted as OSSA by automated systems and OS-MRSA are categorized as CA-MRSA.