Clinical Terminology Systems

Clinical terminology systems provide standardized codes and descriptions for representing health concepts in electronic systems. Using consistent terminologies enables data aggregation, clinical decision support, quality measurement, and interoperable data exchange. The ONC Cures Act Final Rule and US Core Implementation Guide require specific code systems for different data categories.

Comparison Overview

System	Maintainer	Primary Use	Updates	License
ICD-10	WHO / CMS	Diagnosis and procedure coding	Annual	Free (US)
SNOMED CT	SNOMED International	Clinical documentation	Biannual	Free (member countries)
LOINC	Regenstrief Institute	Lab tests and observations	Biannual	Free
RxNorm	NLM	Medication normalization	Weekly	Free
CPT	AMA	Procedure billing	Annual	Licensed (paid)
NDC	FDA	Drug product identification	Continuous	Free

ICD-10-CM / ICD-10-PCS

Maintainer

WHO (ICD-10) / CMS (CM and PCS adaptations)

Purpose

Diagnosis coding (CM) and inpatient procedure coding (PCS). Used for claims, epidemiology, and mortality reporting.

Update Frequency

Annual (October 1 effective date)

Code Format

CM: 3-7 alphanumeric characters (e.g., E11.65). PCS: 7 alphanumeric characters.

Scope

~72,000 diagnosis codes (CM), ~78,000 procedure codes (PCS)

FHIR Binding

Condition.code, DiagnosticReport.conclusionCode

SNOMED CT

Maintainer

SNOMED International (maintained by national release centers)

Purpose

Comprehensive clinical terminology covering diseases, findings, procedures, body structures, organisms, substances, and more. Designed as the universal reference terminology for EHR documentation.

Update Frequency

Biannual international release (January, July). US edition released in March and September.

Code Format

6-18 digit numeric identifiers (e.g., 73211009 for Diabetes mellitus)

Scope

~360,000 active concepts with ~1.5 million relationships

FHIR Binding

Condition.code, Procedure.code, Observation.code, AllergyIntolerance.code

LOINC

Maintainer

Regenstrief Institute

Purpose

Universal codes for laboratory tests, clinical observations, vital signs, surveys, and document types. The standard for identifying what was measured or observed.

Update Frequency

Biannual (June, December)

Code Format

Numeric with check digit, 3-7 characters (e.g., 8480-6 for Systolic blood pressure)

Scope

~100,000 observation terms

FHIR Binding

Observation.code, DiagnosticReport.code, DocumentReference.type

RxNorm

Maintainer

National Library of Medicine (NLM)

Purpose

Normalized names and codes for clinical drugs and drug products. Provides links between drug vocabularies (NDC, GPI, SNOMED CT, MeSH). Used for medication reconciliation, clinical decision support, and e-prescribing.

Update Frequency

Weekly (full monthly release, weekly updates)

Code Format

RxCUI: 5-7 digit numeric identifier (e.g., 197361 for Metformin 500 MG Oral Tablet)

Scope

~120,000 active RxCUIs across ingredient, brand, clinical drug, and pack levels

FHIR Binding

MedicationRequest.medicationCodeableConcept, Medication.code

CPT (Current Procedural Terminology)

Maintainer

American Medical Association (AMA)

Purpose

Procedure codes for medical, surgical, and diagnostic services performed by physicians and other qualified providers. Required for outpatient claims and professional fee billing.

Update Frequency

Annual (January 1 effective date, with quarterly Category III updates)

Code Format

5-digit numeric (Category I), 4 digits + letter (Category III) (e.g., 99213, 0074T)

Scope

~10,000 Category I codes, ~300+ Category III codes

FHIR Binding

Procedure.code, Claim.item.productOrService

NDC (National Drug Code)

Maintainer

FDA

Purpose

Universal product identifier for drugs intended for human use. Identifies the labeler, product, and package size. Used for drug dispensing, inventory management, and claims adjudication.

Update Frequency

Continuous (updated as drugs are registered/deactivated with FDA)

Code Format

10-digit code in 4-4-2, 5-3-2, or 5-4-1 format (e.g., 0002-3227-30)

Scope

~300,000 active NDC codes

FHIR Binding

Medication.code, MedicationDispense.medicationCodeableConcept

FHIR CodeSystem URIs

When using terminology systems in FHIR, each code system is identified by a canonical URI. The following URIs are used in Coding.system to identify the code system.

System	FHIR CodeSystem URI
ICD-10-CM	http://hl7.org/fhir/sid/icd-10-cm
ICD-10-PCS	http://www.cms.gov/Medicare/Coding/ICD10
SNOMED CT	http://snomed.info/sct
LOINC	http://loinc.org
RxNorm	http://www.nlm.nih.gov/research/umls/rxnorm
CPT	http://www.ama-assn.org/go/cpt
NDC	http://hl7.org/fhir/sid/ndc
CVX (Vaccines)	http://hl7.org/fhir/sid/cvx
UCUM (Units)	http://unitsofmeasure.org

Terminology Browsers

Most clinical terminologies provide online browsers for searching and exploring codes: SNOMED CT Browser (browser.ihtsdotools.org), LOINC Search (loinc.org/search), RxNav (rxnav.nlm.nih.gov), ICD-10-CM Tabular Index (icd10data.com), UMLS Terminology Services (uts.nlm.nih.gov).