Radioimmunoassay screening for etorphine in racing horses

A commercially available radioimmunoassay kit was used to screen for the presence of etorphine in post-race urines from horses racing in Kentucky. Most horse urines contained small amounts of materials which reacted positively in this immunoassay. These materials are apparently endogenous to the horse and were called apparent etorphine equivalents. The levels of these apparent etorphine equivalents in post-race urines from 70 horses were estimated. Their modal level averaged 0.1 ng/ml, the population distribution was log normal, and individual horses showed levels of up to 0.8 ng/ml. Dosing horses with etorphine at rates from 1 μg/horse to 100 μg/horse yielded levels of exogenous etorphine equivalents in urine of from 1.0 ng to about 25 ng/ml. These levels of exogenous etorphine equivalents were easily distinguished from background etorphine equivalents values. The radioimmunoassay for etorphine is therefore sufficiently sensitive to allow good control of illegal use of etorphine in racing horses.