Understanding the components, calculation, and impact of monthly and seasonal variation of the standardized antimicrobial utilization ratio (SAAR)

Sean N. Avedissian, Nathaniel J. Rhodes, Jiajun Liu, Doaa Aljefri, Michael J. Postelnick, Sarah H. Sutton, Teresa R. Zembower, David Martin, Gwendolyn Pais, Caroline E. Cruce, Marc H. Scheetz*

*Corresponding author for this work

Research output: Contribution to journalArticle


This study sought to characterize the impact of 3 types of variation on the Standardized Antimicrobial Administration Ratio (SAAR) utilizing local National Healthcare Safety Network (NHSN) data. SAAR and antimicrobial days per 1,000 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 intensive care unit (n 4) and general wards (n 4), the average matched-month percent change in AD/1000DP was correlated with the corresponding change in SAAR (coefficient of determination of 0.99). The monthly mean standard deviation (SD) AD/1000DP was 235 (range, 47.2 to 661.5), and the mean SD SAAR was 1.09 0.26 (range, 0.79 to 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 to 30.69). Eight seasons exhibited seasonal variation in AD/1000DP for oseltamivir with a mean percent change of 129.1% (range, 32.01 to 352.74). The sensitivity analyses confirm that antimicrobial usage at comparator hospitals does not impact the local SAAR, and seasonal variation of antibiotics has the 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.

Original languageEnglish (US)
Article numbere01780-18
JournalAntimicrobial agents and chemotherapy
Issue number3
StatePublished - Mar 2019



  • Antibiotic consumption
  • SAAR
  • Standardized Antimicrobial Utilization Ratio

ASJC Scopus subject areas

  • Pharmacology
  • Pharmacology (medical)
  • Infectious Diseases

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