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.


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.
Payne
Michael A
MA
Environmental Toxicology Extension, University of California, Davis, CA 95616, USA.
Wetzlich
Scott E
SE
Robb
Edward J
EJ
Brown
Scott A
SA
Gardner
Ian A
IA
Cullor
James S
JS
Craigmill
Arthur L
AL
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
D000900:Anti-Bacterial Agents; D002511:Cephalosporins; C053503:ceftiofur