Comparison of animal daily doses and days of therapy for antimicrobials in species of veterinary importance.

Characterizing antimicrobial use in animal populations is critical for purposes of antimicrobial stewardship. While dose-based metrics such as the animal daily dose (ADD) are typically used for such purposes, duration of therapy is emerging as a critical and more intuitive metric. In theory, the number of ADDs should approximate the number of days of therapy (DOTs), but no studies have examined whether this is the case. The objective of this study was to compare antimicrobial ADDs with antimicrobial DOTs in three populations: canine patients, large animal hospital patients, and dairy herds. In the first two populations, dose-based metrics were calculated using administrative hospital records while duration-based metrics were ascertained from manual chart review of individual animals. In the dairy herds, both metrics were obtained via farmer self-report. We found that the correlation between the number of ADDs and DOTs was poor for hospital patients (Lin correlation coefficients of 0.16 and 0.18 for small and large animals, respectively) and that there were often large differences between the two metrics for all populations, with ADDs most often overestimating the number of DOTs. While the median (IQR) differences between the number of DOTs and ADDs were relatively small (-9.4 (-25.7-(-0.92)), 0.34 (-5.0–4.0), and 0.0 (-18.0–9.0) among canine patients, large animal hospital patients, and dairy herds, respectively), the limits of agreement (-89.4–13.2, -37.7–9.9, and -100.0–53.0, respectively) were likely too large to be acceptable for most investigative purposes. Increased discrepancies between the two metrics were significantly associated with certain animal species (e.g., dogs, small ruminants) and drug classes (e.g., penicillins, cephalosporins, macrolides), decreased animal weight, and increased length of hospital stay. While the number of ADDs can approximate the number of DOTs under certain circumstances, the large limits of agreement between these two measurements suggest that the ADD is generally not a reliable proxy for the duration of therapy.