Types of Residues and Their Toxicological Significance

There has been an increasing worldwide public outcry to know what residues and contaminants are in the food supply, and a demand that food be free of residues that could have an impact on the public health. A simplistic but often voiced concept is that edible animal products should be only consumed when all administered drugs and drug-related residues have been totally eliminated. For some time in the past, this concept seemed to guarantee the highest degree of food safety as animal products destined for human consumption were found to be free of drug residues by the analytical methods applied at that time. 

In general, safety evaluation is primarily based on the toxicological testing of the parent drug. However, consumers of edible animal products are also exposed to many other products of drug metabolism, including free metabolites of 

Free metabolites produced by addition, cleavage, oxidation and reduction biotransformations of the parent compound 

Drug fragments incorporated into endogenous cellular components (amino acids, proteins, lipids, etc.) 

The amount of total residues is generally determined by study with radiolabeled drugs and is expressed as the parent drug equivalent in milligrams per kilogram of the food. Bound metabolites can be measured as the difference between the total and extractable residue. Microbiological assays measure the parent molecule and its bioactive metabolites immunochemical assays measure the parent molecule and closely chemically related metabolites. 

---