A MONOCLONAL ANTIBODY-BASED IMMUNOASSAY FOR THE DETECTION OF CEFTIOFUR IN MILK

Ceftiofur is a potent antibiotic used in veterinary medicine. Recently , we developed a sensitive, monoclonal antibody-based competition ELIS A for detecting ceftiofur. In lactating dairy cattle, ceftiofur is use d for the treatment of pneumonia. We report here the application of ou r previously developed ELISA to the analysis of ceftiofur and its meta bolites in milk. In this ELISA, raw milk is simply diluted and added d irectly into the ELISA. Using cefiiofur standards, the immunoassay has a lower limit of detection near I ppb. Matrix effects in milk, howeve r, require that samples be diluted and thus ceftiofur levels below I p pm were not measured. At fortification levels between 25 and 2 ppm, an average recovery of 99.8 +/- 18 was observed. Analysis of incurred re sidues in animals injected daily with therapeutic doses of drug for 5 consecutive days correlated well with studies by others measuring tota l C-14-Ceftiofur residues in animals given the same dose and regime. H igh-performance liquid chromatography (HPLC) analysis of the incurred residue samples only detected a metabolite of ceftiofur-desfuroylcefti ofurcystine-at levels below that reported for ceftiofur equivalents in the ELISA. These data suggest that the ELISA is measuring the ceftiof ur, in addition to its desfuoryl metabolites and conjugates of the des fuoryl metabolite not detected by the HPLC and thus the better correla tion with the earlier radiolabeled experiments of others.