Ampicillin and amoxicillin residue detection in milk, using microbial receptor assay (Charm II) and liquid chromatography methods, after extra-label administration of the drugs to lactating cows
Anderson, K.L.; Moats, W.A.; Rushing, J.E.; Wesen, D.P.; Papich, M.G.
American Journal of Veterinary Research 57(1): 73-78
1996
ISSN/ISBN: 0002-9645 PMID: 8720242 Document Number: 459148
Ampicillin trihydrate or amoxicillin trihydrate was administered at an extra-label dosage of 22 mg/kg of body weight, i.m., once to each of 6 healthy lactating Holstein cows/drug. Milk samples were collected at milkings before and for 156 h after drug administration. Drug concentrations in milk samples from individual cows were determined using the microbial receptor assay method (MRAM; Charm II test) for beta -lactam antibiotics and a liquid chromatography (LC) method with a detection limit of 2 to 5 ppb. Additionally, the classification of milk samples relative to the presence or absence of residues above the FDA tolerance value of 10 ppb was determined. Pharmacokinetic analysis was performed on derived milk drug concentrations. Concentration of ampicillin in milk samples from all cows was <10 ppb by the MRAM and LC methods by the fourth milking (48 h) after treatment with ampicillin. Values were <10 ppb by both methods for all cows treated with amoxicillin by the sixth milking (72 h) after treatment. For individual milk samples, significant differences were found between test methods in the proportion of positive (failing) tests; the MRAM had a higher proportion of presumptive positives. Milk residues >10 ppb were not detected beyond the label milk withholding times for ampicillin (48 h) and amoxicillin (96 h).