Clinical Terminologies & Code Sets

To achieve true interoperability, healthcare systems need more than standardized message formats; they also need a shared vocabulary. Clinical terminologies and code sets define the concepts, lab tests, diagnoses and medications that populate electronic health records. FHIR and other HL7 standards use CodeableConcept and Coding data types to bind data elements to these external code systems. Below are brief overviews of some of the most widely used terminologies, along with their system identifiers used in FHIR and HL7.

SNOMED CT

SNOMED CT (Systematized Nomenclature of Medicine – Clinical Terms) is a comprehensive, multilingual clinical terminology owned and maintained by SNOMED International. It provides concepts and relationships covering symptoms, diagnoses, procedures and findingsnlm.nih.gov. In FHIR, the URI http://snomed.info/sct identifies the SNOMED CT code system. SNOMED concepts are referenced by their numeric concept IDs or compositional expressions, and implementations should specify version URIs when exchanging codes to ensure clarity. SNOMED CT is distributed in national editions; implementers must have appropriate licensing from SNOMED International.

LOINC

LOINC (Logical Observation Identifiers Names and Codes) is a code system managed by the Regenstrief Institute. It assigns universal codes to laboratory tests, measurements and clinical observations. The FHIR system URI for LOINC is http://loinc.org. A typical code might look like 21176‑3, and the display can be taken from the LOINC short or long name. LOINC also defines additional axes—component, property, timing, system, scale and method—which can be exposed as properties in FHIR Terminology Services. The LOINC terminology server is accessible via FHIR APIs.

ICD Codes

The International Classification of Diseases (ICD) is a family of diagnostic code systems maintained by the World Health Organization. ICD‑10 and ICD‑9 are widely used variants; a newer ICD‑11 was released in 2018 and became the official reporting system for member states in 2022. Each revision is treated as a separate code system because their structures differ significantly. FHIR defines canonical URIs such as http://hl7.org/fhir/sid/icd-10 to identify ICD‑10 codes. National variants (e.g., ICD‑10‑CM in the United States) have their own URIs and OIDs. When multiple codes are needed to represent a condition (e.g., dagger–asterisk “dual coding”), FHIR recommends combining the codes in a single string.

Other Code Systems

In addition to the major terminologies above, FHIR and HL7 recognize many other code systems, each identified by a unique URI. Examples include RxNorm for medications, UCUM for units of measure and CPT for procedural codes. FHIR provides a Terminology Service API to validate codes, expand value sets and translate between systems.

How Terminology Binding Works

Both HL7 v2.x and FHIR rely on binding data elements to agreed‑upon code sets, but they do so in slightly different ways:

• HL7 v2.x message fields: In traditional HL7 messages, many fields are designated as “coded” and reference a particular table number. For example, the PID‑8 field (Administrative Sex) refers to HL7 table 0001, which enumerates codes like M, F and U. Tables may be HL7‑defined or user‑defined, and they ensure that senders and receivers interpret codes consistently.

• FHIR resources and profiles: Elements of type CodeableConcept or Coding are bound to value sets. A value set defines the allowed codes from one or more code systems. Each binding has a strength—required, extensible, preferred or example—that indicates how strictly a client must adhere to the specified codes. Servers offer a Terminology Service to validate codes, expand value sets for user interfaces and translate between code systems.

In both paradigms, the goal is the same: to ensure that coded data uses a shared vocabulary so that information can be exchanged and understood unambiguously across systems.

Free Resources

• SNOMED CT Browser – Official browser from SNOMED International for exploring clinical concepts, hierarchies and descriptions.
• LOINC® Database Downloads – Free access to the complete LOINC code set and mapping guides for laboratory and clinical observations.
• ICD‑10‑CM/PCS Resources – U.S. CDC provides free versions of ICD‑10‑CM and ICD‑10‑PCS code sets along with official guidelines.
• UMLS Terminology Services – The Unified Medical Language System from the National Library of Medicine offers cross‑mapping of vocabularies and APIs (free licence required).
• OHDSI Athena – Open‑source repository of standard clinical vocabularies (SNOMED, LOINC, RxNorm, etc.) with tools for downloading and mapping.