Comparison of the use of regulatory assays and high-performance liquid chromatography for detection of residues of ceftiofur sodium metabolites in tissue specimens of culled dairy cattle

Objective - To compare the results of regulatory screening and confirmation assays with those of high-performance liquid chromatography (HPLC) in the detection of ceftiofur metabolites in the tissues of culled dairy cattle. Animals - 17 lactating Holstein dairy cows. Procedure - Daily IM injections of ceftiofur sodium were administered at a dose of 2.2 mg of ceftiofur equivalents/kg (n=6) or 1.0 mg of ceftiofur equivalents/kg (10) for 5 days. Following withdrawal times of 12 hours (high-dose ceftiofur) and either 5 or 10 days (low-dose ceftiofur), cows were slaughtered and liver, kidney, and diaphragmatic muscle specimens were harvested and analysed by HPLC and standard regulatory methods that included the following assays: the swab test on premises, the fast antimicrobial screen test, the calf antibiotic and sulfa test, and the 7-plate bioassay confirmation test. Results - In all tissue specimens, residues of ceftiofur and desfuroylceftiofur-related metabolites, as measured by HPLC, were less than regulatory tolerance, as defined by the FDA. False-positive screening assay results were more likely for tissue specimens that had been frozen for shipment to a federal laboratory, compared with fresh tissue specimens that were assayed at the slaughter establishment (23% vs 3% false-positive results, respectively). Conclusions and Clinical Relevance - The observation that fresh tissues had negative results on screening assays, whereas subsets of the same tissue specimens had false-positive results on screening assays following freezing, suggests that freezing and thawing interferes with microbial inhibition-based regulatory screening assays.