An enzyme linked immunosorbent assay (ELISA) for residues of monensin in plasma of cattle

An ELISA method, many times more sensitive than previous methods, was developed to measure residues of monensin in bovine plasma. Rabbit antibodies to monensin were allowed to compete for monensin immobilised on microtitre plates and residues in solution. The antibody bound to the wells was measured by incubating a commercially available anti-rabbit antibody labelled with alkaline phosphatase and measuring the enzyme activity bound to the first antibody. Standard curves were constructed over 0–1 ng monensin. 100 μl plasma was used in the assay. Three steers were dosed daily per os with the recommended doses of monensin for 16 days. Residues of monensin (ca. 25 μg/ l) were detected in plasma throughout the treatment period and for a further 5 days after the withdrawal of the drug. It was estimated that residues cleared bovine plasma in about 8 days. Although the sensitivity of the assay is less than 1 μg/l a limit of decision of 5 μg/l is suggested for monitoring purposes and animals receiving monensin would be positively identified using this test.