The use of a specific enzyme‐linked immunosorbent assay to monitor the persistence of penicillin g residues in milk following intramammary antibiotic treatment

Lactating dairy cows were treated with a single intramammary infusion of a commercial penicillin G‐dihydrosteptomycin formulation commonly used for the treatment of mastitis. The levels of penicillin G excretion were measured directly in the raw milk using a specific competitive enzyme‐linked immunosorbent assay. Clearance of penicillin residues (<3–0 ng, 0–005 IU per ml) was seen between four and nine milkings for individual animals with 68% of the injected antibiotic being excreted intact. The dynamic range of the assay was between 3 and 96 ng/ml, the mean intra‐assay variation was 5–3% and interassay variation was 5–4% at the limit of quantification.