liminfoSNOMED CT Reference
Free reference guide: SNOMED CT Reference
About SNOMED CT Reference
The SNOMED CT Reference is a searchable guide to the Systematized Nomenclature of Medicine - Clinical Terms, the most comprehensive clinical terminology system with over 350,000 active concepts. This reference covers the fundamental three-component structure of SNOMED CT: Concepts (unique numeric identifiers like 73211009 for Diabetes mellitus), Descriptions (human-readable terms including Fully Specified Names, Preferred Terms, and Synonyms), and Relationships (defining connections between concepts).
The reference details all major relationship types used in SNOMED CT, including IS-A hierarchies for taxonomic classification with support for multiple inheritance, FINDING_SITE for anatomical locations of disorders, CAUSATIVE_AGENT for disease-causing substances, ASSOCIATED_MORPHOLOGY for pathological changes, and METHOD for procedural techniques. It also covers the 19 top-level hierarchies from Clinical finding and Procedure to Body structure and Pharmaceutical product.
For practical implementation, this reference includes Expression Constraint Language (ECL) syntax for querying concept sets using descendant operators (<< and <), ancestor operators (>> and >), and attribute-based constraints. It covers post-coordination for combining concepts, Reference Sets (Refsets) for concept grouping and external code mapping (such as ICD-10), RF2 distribution format file structures, and integration with FHIR resources and the Snowstorm terminology server API.
Key Features
- Complete SNOMED CT concept structure guide including SCTID format with Verhoeff check digit validation
- Relationship type reference covering IS-A, FINDING_SITE, CAUSATIVE_AGENT, ASSOCIATED_MORPHOLOGY, and METHOD
- All 19 top-level hierarchy descriptions with key sub-hierarchies and example concept IDs
- Expression Constraint Language (ECL) syntax guide with descendant, ancestor, and attribute constraint operators
- Post-coordination expression examples for combining concepts to represent complex clinical meanings
- Reference Set (Refset) types explained: Simple, Language, Simple Map, and Attribute Value refsets
- ICD-10 mapping reference with 1:1 and rule-based 1:N mapping examples
- FHIR integration patterns and Snowstorm REST API endpoint reference for terminology server queries
Frequently Asked Questions
What is a SNOMED CT Concept ID (SCTID)?
An SCTID is a unique 6-18 digit numeric identifier assigned to every concept in SNOMED CT. For example, 73211009 represents Diabetes mellitus and 22298006 represents Myocardial infarction. The last digit is a Verhoeff check digit for error detection. Each concept can have multiple descriptions (terms) but only one unique SCTID.
What is the difference between a Fully Specified Name and a Preferred Term?
A Fully Specified Name (FSN) is the unambiguous name that includes a semantic tag in parentheses, such as "Diabetes mellitus (disorder)." The Preferred Term is the clinically preferred display name without the semantic tag, like "Diabetes mellitus." Concepts can also have Synonyms (e.g., "DM" or "Sugar diabetes") that are alternative human-readable terms.
How does the IS-A hierarchy work in SNOMED CT?
IS-A is the fundamental taxonomic relationship creating parent-child hierarchies. For example, Diabetes mellitus IS-A Disorder of glucose metabolism IS-A Metabolic disease IS-A Disease. Importantly, SNOMED CT supports multiple inheritance, meaning a concept can have more than one parent, allowing it to be classified along multiple axes simultaneously.
What is Expression Constraint Language (ECL)?
ECL is a formal query language for defining sets of SNOMED CT concepts. The << operator selects all descendants including the concept itself (e.g., << 73211009 for all types of diabetes), while < excludes the concept itself. You can also filter by attributes, such as finding all clinical findings with a heart-related FINDING_SITE using attribute constraints.
How do I map SNOMED CT codes to ICD-10?
SNOMED CT provides official mapping reference sets to ICD-10. Some mappings are simple 1:1 (e.g., 73211009 Diabetes mellitus maps to E14), while others require rule-based 1:N mappings where multiple ICD-10 codes may apply depending on context. The mappings are distributed as part of the SNOMED CT release in Simple Map Refset format.
How is SNOMED CT used in FHIR resources?
In FHIR, SNOMED CT codes are used within CodeableConcept elements with the system URI "http://snomed.info/sct". For example, a Condition resource would include the code 73211009 with display "Diabetes mellitus". SNOMED CT can be used in Condition, Procedure, Observation, MedicationRequest, and many other FHIR resource types.
What is the Snowstorm terminology server?
Snowstorm is the official open-source SNOMED CT terminology server maintained by SNOMED International. It provides REST APIs for concept lookup (GET /browser/{branch}/concepts/{id}), text search (GET /browser/{branch}/concepts?term=diabetes), hierarchy navigation, and ECL query execution. It can be deployed locally or accessed via hosted instances.
What is post-coordination in SNOMED CT?
Post-coordination allows combining existing SNOMED CT concepts to express clinical meanings that do not have a pre-coordinated (existing) concept. For example, you can combine "Fracture of bone" with a FINDING_SITE of "Femur" to represent a femoral fracture. This uses SNOMED CT expression syntax with concept IDs and attribute-value pairs.