Towards an improved estimate of antimicrobial use in animals: Adjusting the "population correction unit" calculation

International comparisons of animal antimicrobial use (AMU) have typically been based on total national estimates of antimicrobials sales standardized by the national animal biomass calculated as the population correction unit (PCU). The objective of this paper was to compare the currently accepted PCU calculation with that of the adjusted population correction unit (APCU), which re-evaluates the standard animal weights used in the calculation and accounts for animal lifespan. The APCU calculation resulted in substantial changes to the 2009 national biomass estimates for cattle, pigs, and poultry in 8 European countries and Canada. The estimated national biomass for cattle increased 35% to 43%, while the estimated national biomass of pigs and poultry typically decreased by approximately 51% and 87%, respectively. Among the 9 countries, the total national APCU ranged from an increase of 1% to a decrease of 40% relative to PCU, and these differences were statistically significant. Adjusted population correction unit is preferred over PCU in comparing and contrasting AMU among animals with different lifespans because it is more transparently derived and is a reasonable approximation of the animal biomass at risk of antimicrobial treatment.