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To determine percentage of false-positive test results for assays used by regulatory agencies to detect antibiotic residues in tissues.


Prospective study.


426 dairy cows.


Dairy cows scheduled for culling that were identified as being unlikely to have antibiotic residues in tissues on the basis of strict inclusion criteria were used. A sample of kidney obtained from each cow at slaughter was tested on-site, using the swab test on premises (STOP; 97 samples) or the fast antibiotic screening test (FAST; 329 samples). Frozen samples (n = 1,278) of liver, muscle, and kidney were thawed and retested at a federal laboratory, using the same screening assays. Kidney and liver samples (n = 852) were also tested using the 7-plate bioassay confirmation test used for confirmation and identification of antibiotic residues.


Results of screening assays performed onsite were negative. When frozen samples were retested, 20 (12 liver, 7 kidney, and 1 muscle) had positive FAST results, but none had positive STOP results. Of the samples tested with the 7-plate bioassay confirmation test, 4 liver samples had results indicating a tetracycline (n = 3) or an unidentified microbial inhibitor (1) as a residue.

Clinical Implications

Results suggest it is unlikely that regulatory action will be taken against producers sending untreated cattle to market. However, because results of the FAST and 7-plate bioassay confirmation test were positive when applied to frozen tissue, use of assays based on microbial inhibition may not be valid for confirmation of residues. (J Am Vet Med Assoc 1999;214:1048–1050)

Free access
in Journal of the American Veterinary Medical Association