title: 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.
abstract: 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.
  
  
  METHODS
  17 lactating Holstein dairy cows.
  
  
  METHODS
  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 analyzed 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
  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.
authors.affiliation: Environmental Toxicology Extension, University of California, Davis, CA 95616, USA.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Payne
authors.forename: Scott E
authors.initials: SE
authors.lastname: Wetzlich
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Robb
authors.forename: Scott A
authors.initials: SA
authors.lastname: Brown
authors.forename: Ian A
authors.initials: IA
authors.lastname: Gardner
authors.forename: James S
authors.initials: JS
authors.lastname: Cullor
authors.forename: Arthur L
authors.initials: AL
authors.lastname: Craigmill
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000032:analysis*; D002417:Cattle / Q000378:metabolism*; D002511:Cephalosporins / Q000032:analysis*; D002851:Chromatography, High Pressure Liquid / Q000662:veterinary*; D004350:Drug Residues / Q000032:analysis*; D005260:Female; D007668:Kidney / Q000187:drug effects / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D002511:Cephalosporins; C053503:ceftiofur