A method of classifying each individual diagnosis and treatment in the NHS for entry into a data repository, held either at the institution treating the patients or nationally. The classification is undertaken by coders who extract the relevant information from medical notes. In the UK, the codes are those defined in the international classification of diseases handbook (ICD-10) for symptoms and diagnosis and the Office of Population Censuses and Surveys list of procedures (OPCS-4). As electronic records become universal, clinicians will take over responsibility for coding using a language called SNOMED CT which offers a unique identification number, name and preferred term for every disease concept. The data are ultimately used for audit, research, service development and quality improvement.