Measuring antimicrobial use on dairy farms: a longitudinal method comparison study

Antimicrobial use on UK dairy farms is measured for surveillance purposes and utilizes veterinary sales data as a proxy for use. Two other methods of recording use have been used commonly on-farm: medicine waste bins and farm medicine records. However, none of these methods have been validated to measure antimicrobial use. The objectives of this research are to assess agreement between the 3 most common methods for measuring on-farm antimicrobial use with a pre-determined "gold standard" measure. Antimicrobial use was measured prospectively on 26 UK dairy farms using medicine waste bins into which participants placed all discarded medicine packaging for a 12-month period. At the end of 12 months, farm medicine records and veterinary sales data were obtained retrospectively for participating farms. The systematic difference between the mean on-farm antimicrobial use measured by each of the 3 methods with a gold standard measure was investigated using one-way repeated measures ANOVAs. Reliability and clinical relevance of the agreement between each pair of methods was quantified using the concordance correlation coefficient and the Bland-Altman method, respectively. Veterinary sales data shows excellent reliability for all forms of antimicrobial when compared with the gold standard. Medicine waste bins show moderate to excellent reliability for injectables, poor to good reliability for intramammaries and no agreement for other forms of antimicrobial. Farm medicine records do not show agreement for any form of antimicrobial when compared with the gold standard. The use of veterinary sales data as a proxy for on-farm antimicrobial use in the UK represents excellent statistical reliability and offers a clinically acceptable agreement with a gold standard method when used to measure both injectable antimicrobials and intramammary antimicrobials. These results have policy implications both nationally and internationally and are essential in quantifying the actual impact of agricultural antimicrobial use on both animal and human health.