Determination of dexamethasone in milk of dairy cows by immuno-enzymatic assay

Eight lactating cows received 3 im injections at 24 h intervals of a commer cial formulation containing dexamethasone. Each treatment provided 25 mug/k g bw/d of dexamethasone acetate, equivalent to 22.6 mg of dexamethasone. Mi lk sample were obtained before treatment (5 d), during the treatment period , and for up to 22 milking after the last injection. The concentrations of dexamethasone in the milk samples were determined by a commercial competiti ve immunoenzymatic assay for corticosteroids (detection limit 0.15 ng dexam ethasone/ml). The conventional therapeutic dose of dexamethasone acetate ca used milk drug concentrations exceeding the tolerated maximum residue limit (0.3 mg/kg). A withdrawl time of 3-3.5 d for dexamethasone in milk provide d sufficient protection for consumer health. The commercial enzyme immunoas say kit employed in this study was sufficiently sensitive, easy to use, and appropriate to monitor the use of dexamethasone in lactating animals.