(E29) Comparison of Equine Antimicrobial Susceptibility Based on Minimum Inhibitory Concentration and Pharmacokinetic-Pharmacodynamic Indices for Enrofloxacin

Abstract:

Background: Culture & Susceptibility (C&S) and minimum inhibitory concentration (MIC) testing commonly guide antimicrobial selection, but incorporating pharmacokinetic-pharmacodynamic (PK-PD) indices better predicts in vivo efficacy. For fluoroquinolones, optimal bactericidal activity occurs at an area under the curve (AUC):MIC ratio ≥125 or maximum (peak) plasma concentration (Cmax):MIC ratio ≥10, enhancing bacterial killing and reducing resistance. Despite this, MIC-based C&S results frequently guide treatment without PK-PD consideration.

Hypothesis/

Objectives: This study aimed to assess agreement between MIC-based and PK-PD-derived susceptibility across enrofloxacin doses and routes. We hypothesized MIC-based classifications would poorly align with PK-PD predictions.

Methods: C&S and MIC results from clinical records were retrospectively reviewed. PK-PD-derived susceptibility was assessed using AUC and Cmax values from published enrofloxacin PK studies in adult horses. Agreement between MIC-based and PK-PD-derived susceptibility was evaluated using descriptive and Cohen’s kappa statistics.

Results: C&S testing classified 461 isolates (90.2%) as susceptible and 50 (9.8%) as resistant to enrofloxacin. Agreement between MIC-based and PK-PD-derived susceptibility varied by dose and route. At 2.5 mg/kg, intragastric (IG) administration showed 0.11 agreement, while intravenous (IV) reached 0.36. At 5 mg/kg, IV agreement ranged from 0.25 to 1. Intramuscular administration achieved 0.64, and IG 0.82. At 7.5 mg/kg, the recommended per oral (PO) dose, agreement was 0.36.

Conclusions and Clinical Importance: MIC-based susceptibility showed variable agreement with PK-PD derived susceptibility, with only fair agreement for the recommended PO dose. Integrating PK-PD indices for treating bacterial infections is crucial for optimizing efficacy and minimizing resistance against enrofloxacin in equine practice.