Chloramphenicol stereoisomers need to be distinguished: consequences observed from a proficiency test

Although the use of chloramphenicol (CAP) as a veterinary drug is banned in the European Union and many other countries, monitoring for CAP residues in food is routine. Positive detections are few, but taken extremely seriously. European Union laboratories analysing for CAP should validate methods according to European Commission Decision 2002/657/EC, must be accredited to ISO 17025, and will generally participate in proficiency testing (PT) schemes, such as those offered by the Food Analysis Performance Assessment Scheme (FAPAS®). The FAPAS PTs aim to cover a wide range of relevant matrices including honey, prawns, fish, milk and kidney. Test materials are prepared either by animal dosing studies or by spiking raw matrix. The most common method reported by FAPAS participants used to screen for CAP residues is LC-MS/MS, but ELISA kits are increasingly being used. A recent PT round highlighted that the result obtained might be correlated with the type of analytical method being employed. Follow-up investigations have demonstrated that some of these variations in data are a function of the different stereoisomeric forms of CAP. This paper discusses the implication of this research on method validation requirements and European Union legislation.