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Revision of a European Union official method to enforce legal provisions: The case of diclazuril

Diclazuril is a coccidiostat currently authorized as feed additive in the European Union (EU), with a legal limit set at 1 mg/kg. For official control, an official EU method based on reversed-phase HPLC coupled with UV detection at 280 nm needs to be applied. Recently, the EU Reference Laboratory for feed additives was informed that the recovery rate for diclazuril was very low when implementing this method and performed experiments demonstrating that the indicated sorbent mass of the solid-phase extraction (SPE) was too low. Objective: Therefore, the paper presents a modified method protocol and the results of an interlaboratory study, performed on two compound feedingstuffs containing diclazuril around the legal limit. Methods: The official method was modified by using a higher SPE sorbent mass and was further subjected to validation. Results: The obtained values for the relative standard deviation for repeatability were 4.5 and 11.2%, and the corresponding values for the relative standard deviation for reproducibility were 14.3 and 18.1%; the calculated HorRat values were 0.95 and 1.14. Furthermore, acceptable mean recovery values of 98 and 111% were obtained for the two test materials, respectively. Conclusions: Based on the obtained performance profile, it was concluded that the modified official method was fit for purpose. In consequence, the official EU method will be corrected accordingly. Highlights: The highlights of this work are reflected by the following terms, namely Diclazuril, correction of EU official method, and interlaboratory study.