DISCUSSION

To our knowledge, this is the first comparative description of 2 denominators used to represent patient time at risk for antimicrobial use. Patient days, the traditional infection prevention denominator that counts at a single time each calendar day, may miss a partial day at risk on the day of admission or discharge, depending on the time of the daily census count. The newer days-present metric attempts to address this by counting all partial days. When aggregated, the additional time resulting from partial days increased AU rate denominator counts substantially. In our analysis, days present counts were approximately one-third higher than person time rounded to the nearest minute. Relative differences varied among hospitals and units and was highly correlated to length of stay.

Our findings have important implications for AU assessments. First, AU estimates using days present will be substantially lower than those using patient days. Thus, stewards need to carefully delineate days present versus patient days when interpreting and time-trending local data and when comparing local AU estimates to published literature or publically available AU estimates. Similarities in these terms and abbreviations may cause confusion. Second, the impact of short-stay patients has implications for hospital and unit comparisons. The presence of extra time in aggregated days present estimates will result in lower AU estimates in locations that care for patients with short stays. High-volume units with short stays (eg, labor and delivery wards and nurseries) have been considered lower-risk areas for antimicrobial exposure, but these had the highest RDs in our study. Stenejhem et alReference Stenehjem, Hersh and Sheng ¹⁰ described how the inclusion of these “miscellaneous” units inflated facility-wide denominators and affected the utility of facility-level comparisons; ultimately, they decided to exclude those units when benchmarking. Similarly, the NHSN AU SAARs (standardized antimicrobial administration ratios) exclude all units except general medical/surgical wards and medical/surgical intensive care units. ⁷ Eliminating units from analyses limits information gained about these patient populations and excludes them from assessment for improvement opportunities. Third, we observed that the RDs between patient days and days present varied among hospitals and that academic hospitals had lower RDs. We hypothesize that this observation is related to complex case mix and its association with longer length of stay. Risk adjustment methods for hospital benchmarking may help improve comparisons; however, the large effect from length of stay may be difficult to fully overcome with risk adjustment.

The advantages of the days present metric include the ability to participate in the NHSN AU option and access national data for comparisons. Alternatively, patient days are readily available, actively used by infection prevention, and do not require additional resources from information technology (Table 2). Individual hospitals should choose a single AU denominator metric based on their available resources and needs, then standardize terminology to most effectively interpret and externally share analyses of ASP impact.

TABLE 2

Antimicrobial Use Denominator Metric Summary

NOTE. NHSN, National Healthcare Safety Network; AU antimicrobial use.

This study has several limitations. The 7-hospital study sample may not be large enough to fully describe the comparisons. Included hospitals are in the southeastern United States and may differ from hospitals in other geographic locations and practice settings. Existing data within the NHSN could be used to validate these findings.

In summary, days present denominators increased days at risk estimates by approximately one-third when compared with patient days. This effect differed among hospitals and units and was highly influenced by short length of stay.