Diversity of Antimicrobial Use and Resistance in 42 Hospitals in the US
Study Objective: To measure diversity (or heterogeneity) of antibiotic use in a sample of hospitals in the United States and to assess an association with bacterial resistance.
Design: Observational cross-sectional study.
Setting: Forty-two general medical-surgical hospitals, mostly in the Eastern United States.
Data Source: Administrative claims data and hospital antibiograms during 2003.
Measurements and Main Results: Antibiotic use was measured by defined daily dose (DDD), and diversity was assessed by using Simpson's Index and the Shannon-Weiner Index. Aggregated antibiograms (from 17 hospitals) were used to assess resistance. There were a total of 3,655,579 patient-days during 2003, with a mean ± SD of 87,037 ± 62,679 patient-days/hospital. Mean ± SD antibiotic use was 704 ± 244 DDD/1000 patient-days/hospital, and fluoroquinolones represented the largest single class. Mean ± SD diversity by Simpson's Index was 0.861 ± 0.022/hospital, and a strong correlation was noted between the two diversity measures. Multivariate analysis found no significant relationship between diversity and the proportion of resistant pathogens.
Conclusion: Diversity of aggregate antibiotic use can be measured, but additional investigations are needed to determine if it is a useful strategy to contain resistance.

Most efforts to improve antimicrobial use in hospitals have focused on reducing expenses. More recently, efforts have shifted the desired outcome from cost control to a delay or reduction in the emergence of bacterial resistance. One proposed strategy to delay the emergence of resistance in hospitals is usually referred to as diversity, or heterogeneity, of antibiotic use (also called "mixing"). Mathematic models predict that diversity will be more effective than cycling in reducing the evolution or spread of antibiotic resistance. The concept behind diversity is that the selective pressure leading to amplification and cross-transmission of resistant pathogens will decrease when a mix of antimicrobials with different mechanisms of action are used throughout the hospital. The potential value of this strategy has been summarized as follows: "there is reason to believe that clinical strategies that encourage heterogeneity of antibiotic use will help to stabilize, if not entirely prevent, the selection of resistance." Two pilot studies in Spanish intensive care units (ICUs) compared a diversity strategy with cycling and reported mixed results. Recent antimicrobial stewardship guidelines from the Infectious Diseases Society of America (IDSA) and the Society for Healthcare Epidemiology of America (SHEA) have recognized the need for clinical investigations to assess the value of diversity.

The purpose of this study was to measure diversity of antibiotic use in a sample of hospitals in the United States and to assess an association with bacterial resistance.