Homogeneity of antimicrobial policy, yet heterogeneity of antimicrobial resistance: antimicrobial non-susceptibility among 108,717 clinical isolates from primary, secondary and tertiary care patients in London

Objectives: We examined the four-year trend in antimicrobial susceptibilities and prescribing across levels-of-care at two London teaching hospitals and their multisite renal unit, and for the surrounding community.
Methods: Laboratory and pharmacy information management systems were interrogated, with antimicrobial use and susceptibilities analysed between hospitals, within hospitals, and over time.
Results: 108,717 isolates from 71,687 patients were identified, with significant differences (at p<0.05) in antimicrobial susceptibility between and within hospitals. Across the four years, rates of extended-spectrum β-lactamase (ESBL)-/AmpC-producing Enterobacteriaceae ranged from 6.4 to 10.7% among community isolates, 17.8 to 26.9% at ward level and 25.2 to 52.5% in critical-care. Significant variations were also demonstrated in glycopeptide-resistant enterococci (ward level 6.2 to 17.4%; critical-care 21.9 to 56.3%), methicillin-resistant Staphylococcus aureus (ward level 18.5 to 38.2%; critical-care 12.5 to 47.9%) and carbapenem-resistant Pseudomonas spp. (ward level 8.3 to 16.9%; critical-care 19.9 to 53.7%). Few instances of persistently higher resistance were seen between the hospitals in equivalent cohorts, despite persistently higher antimicrobial use in hospital 1 than hospital 2. We found significant fluctuations in non-susceptibility year-on-year across the cohorts, but with few persistent trends.
Conclusions: The marked heterogeneity of antimicrobial susceptibilities between hospitals, within hospitals, and over time demands detailed, standardised surveillance and appropriate benchmarking to identify possible drivers and effective interventions. Homogenous antimicrobial policies are unlikely to continue to be suitable as individual hospitals join hospital networks, and policies should be tailored to local resistance rates, at least at the hospital level, and possibly with finer resolution, particularly for critical-care.