Application of total error approach to assess the performance of a biological method (ELISA) to detect nicarbazin residues in eggs

Nicarbazin, a coccidiostat, is used as a feed additive in poultry but not in laying hens. Feed contamination may however occur resulting in residues being present in eggs. As a Maximum Residue Limit (MRL) does not exist for nicarbazin residues in eggs a “Differential Action Level” (DAL) of 100 μg/kg has been established by the Veterinary Medicines Directorate (VMD). We have studied a commercial ELISA kit validated to detect and quantify nicarbazin in eggs with a sensitivity of 3 μg/kg. We used the total error approach to assess the performance of and validate the kit at the DAL level. The accuracy profile has been successfully obtained for the ELISA kit. The method cannot however be validated as a semi-quantitative method and we have consequently determined a cut-off based on 5% false negative rate according to European Decision 2002/657 on blank and spiked samples (70 μg/kg). The cut-off value established was 20 μg/kg using the 95th percentile.