Understanding the components, calculation and impact of monthly and seasonal variation of the SAAR: Illustrative Data [Epidemiology and Surveillance]

This study sought to characterize the impact of 3 types of variation on the Standardized Antimicrobial Administration Ratio (SAAR) utilizing local NHSN data. SAAR and antimicrobial days per 1000 days present (AD/1000DP) were compiled monthly for Northwestern Memorial Hospital from 2014 to 2016. Antimicrobial consumption was aggregated into agent categories (via NHSN criteria). Month-to-month changes in SAAR and AD/1000DP were evaluated. Azithromycin and oseltamivir AD/1000DP from 2012 through 2017 were explored for seasonal variation. A sensitivity analysis was performed to explore the effect of seasonality and altered consumption at other hypothetical hospitals on the SAAR. Across agent categories for both the ICU (n=4) and general wards (n=4), the average matched-month percent change in AD/1000DP was correlated with the corresponding change in SAAR (R² = 0.99). The monthly mean±SD AD/1000DP was 235 (range 47.2-661.5), and the mean±SD SAAR was 1.09±0.26 (range 0.79-1.09) across the NHSN agent categories. Five seasons exhibited seasonal variation in AD/1000DP for azithromycin with a mean percent change of 26.76% (Range: 22.27-30.69). Eight seasons exhibited seasonal variation in AD/1000DP for oseltamivir with a mean percent change of 129.1%, (Range: 32.01-352.74). The sensitivity analyses confirm that antimicrobial usage at comparator hospitals does not impact the local SAAR, and seasonal variation of antibiotics have potential to impact SAAR. Month-to-month changes in the SAAR mirror monthly changes in an institution's AD/1000DP. Seasonal variation is an important variable for future SAAR consideration, and the variable antibiotic use at peer hospitals is not currently captured by the SAAR methodology.

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