Investigation into the experimental protocols required to determine maximum residue limits (MRLs) in honey

There is current debate within the EU, and internationally, on how withdrawal periods and maximum residue limits (MRLs) may be set for honey production. Whilst comprehensive EU guidelines exist for calculating the withdrawal times of veterinary medicines in most food-producing species, the analytical variables to be studied for bees/honey are not well defined. The objective of this study was therefore to investigate and understand the factors, for example sampling variability, that is important in the development of a harmonized protocol that can be used to generate the robust scientific data necessary to assist risk assessors in proposing MRLs for honey. Ten bee colonies were treated in the spring with a model compound (ciprofloxacin). One hive was used to study intra-hive variation in residue concentrations and the other nine were used in an inter-hive study over a 41-week sampling period. All samples were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The highest mean concentration from nine hives used in the inter-hive study was 4627 µg/kg eight days (D8) after treatment. The concentration of ciprofloxacin declined to an average concentration of 1756 µg/kg at D30 and 733 µg/kg at D283 (over-winter sample). A generalized additive model was used to fit a smooth curve for trend estimation. For some individual hives the concentration of ciprofloxacin increased slightly at the later sampling time-points. Consequently it was not possible to interpolate, with confidence, a finite withdrawal period for ciprofloxacin at theoretical MRLs between 25 and 500 µg/kg. The observed variation in concentration of ciprofloxacin between hives indicates that the validity of the EU guideline for bees/honey, which requires five samples from five hives to calculate a withdrawal period, may require revision.