A classification groups diseases and other health issues into categories based on similar properties. These categories may group by type of disease, body system or anatomy, are often hierarchical (e.g. acute through to chronic illness) and are ‘exhaustive’ which means almost all terms can be classified through use of categories like NEC ‘not elsewhere classified’. A classification has rules and standards to be followed when assigning codes, and often has an index directing to the correct code assignment. Classifications are used for statistics, epidemiology, audit, planning, financial billing and other use cases where amalgamated data is required. ICD-10 and ICPC are classifications as they have the characteristics noted above.
A terminology is set of terms representing the system of concepts in a particular field. SNOMED CT is an international clinical reference terminology designed for use in electronic health records and other electronic health recording. SNOMED CT is a structured list of health terms (known as concepts) that have definitions and relationships. Unlike a classification, a terminology allows the clinician to use natural medical vocabulary, and the defining relationships such as the position in the SNOMED CT tree hierarchy determine the meaning. Compared to a classification, where elements are grouped into limited categories, SNOMED CT is large and comprehensive but can be broken down into specialty reference sets. Use of SNOMED CT in an electronic health record structures health data that may then be converted into meaningful information and manipulated automatically. SNOMED CT terms can be linked by mapping to classifications like ICD-10 and ICPC.