Optical immunobiosensor assay for determining enrofloxacin and ciprofloxacin in bovine milk.

A rapid, sensitive optical immunobiosensor assay was developed and used to determine enrofloxacin and its main metabolite, ciprofloxacin, in milk from healthy cows and cows with clinical signs of mastitis after intramuscular administration of enrofloxacin. Liquid chromatography (LC) was used to confirm results of the biosensor assay. Despite incomplete cross-reactivity between polyclonal enrofloxacin antibodies and ciprofloxacin, the biosensor assay could be used for semiquantitative analysis of the sum of the 2 substances. LC analysis showed that ciprofloxacin persisted at levels exceeding the expected future maximum residue limit in milk for several days after the end of the withdrawal period.