Abstract: A method of monitoring a therapeutic regimen in an animal, the method involving a) providing to the animal a therapeutic compound and a detectable compound that passes into the bloodstream, excretory system, or other tissue or body fluid in detectable form; b) after a period of time, following step (a), sufficient for passage of a detectable amount of the detectable compound into the body fluid or tissue, collecting a sample of the fluid or tissue from the animal; and c) measuring or detecting the detectable compound, or a detectable metabolite thereof, in the sample, wherein the detectable compound involves one member of a specific binding pair, and detection is carried out using the second member of the specific binding pair.