Short answer: SNOMED CT is a clinical terminology: more than 360,000 active concepts that let a clinician record exactly what they observed, with as much detail as they want, in a web of relationships (a fracture of the left femoral neck caused by a fall). ICD-10 is a statistical classification: about 14,000 categories, each a box into which cases are sorted so that countries can count deaths and diseases and hospitals can bill (S72.0, fracture of neck of femur). Terminologies capture; classifications count. A modern record uses SNOMED CT for care and maps to ICD-10 for reporting, which is why the two appear side by side in a FHIR Condition.
Side by side
Side by side, SNOMED CT is a clinical terminology owned by SNOMED International, with about 360,000 active concepts in a polyhierarchy, built to record clinical meaning at the point of care. ICD-10 is a statistical classification owned by the World Health Organization, with about 14,000 codes in a single hierarchy, built to count diseases and deaths and to support reimbursement.
| SNOMED CT | ICD-10 | |
|---|---|---|
| Owner | SNOMED International (member countries) | World Health Organization |
| Purpose | Record clinical meaning at the point of care | Classify diagnoses for statistics, epidemiology, reimbursement |
| Size | ~360,000 active concepts, ~1.5 million descriptions | ~14,000 codes (WHO); 70,000+ in ICD-10-CM |
| Structure | Polyhierarchy: a concept has many parents; defined by relationships | Single hierarchy: chapters → blocks → categories → subcategories |
| Identifier | Numeric concept ID, e.g. 44054006 | Alphanumeric code, e.g. E11.9 |
| Granularity | As fine as needed; post-coordination for more | Fixed; residual categories ("other", "unspecified") |
| Covers | Findings, disorders, procedures, body structures, organisms, substances, observables, social context | Diseases, injuries, causes of death, reasons for contact (chapter Z) |
| Updates | Twice a year international; national extensions | WHO annual updates; ICD-11 since 2022 |
| Licence | Free in member countries; affiliate licence elsewhere | Free (WHO); national variants have their own terms |
| In FHIR | system http://snomed.info/sct | system http://hl7.org/fhir/sid/icd-10 (and national variants) |
For each on its own, see What is SNOMED CT? and ICD-10 vs ICD-11.
Why the mapping goes one way
The mapping between SNOMED CT and ICD-10 goes one way because SNOMED CT is more detailed than ICD-10. Every SNOMED CT disorder can be assigned to an ICD-10 category through the official SNOMED CT to ICD-10 map, but one ICD-10 code covers many SNOMED CT concepts, so the detail cannot be recovered in the reverse direction.
Every SNOMED CT disorder can be assigned to an ICD-10 category: SNOMED International publishes the SNOMED CT to ICD-10 map, which gives, for each concept, the target code and the rules when context matters (age, sex, a second code for the cause). The reverse does not work: ICD-10 E11.9 (type 2 diabetes without complications) covers dozens of SNOMED concepts, and the code alone cannot tell you which one. Information flows from the detailed terminology to the coarse classification, never back.
In practice, a clinician picks a SNOMED CT concept from a search; the system proposes the ICD-10 code from the map; a coding professional confirms or adjusts it against national coding rules, which may require extra codes (laterality, external cause, complications) that SNOMED expresses in one concept.
Dual coding in a FHIR Condition
Dual coding in a FHIR Condition means placing a SNOMED CT coding and an ICD-10 coding in the same Condition.code CodeableConcept, one for clinical meaning and one for reporting. Both describe a single concept; profiles decide which is required, the IPS asking for SNOMED CT where available and billing systems for the national ICD variant.
{
"resourceType": "Condition",
"clinicalStatus": {"coding": [{"system": "http://terminology.hl7.org/CodeSystem/condition-clinical", "code": "active"}]},
"code": {
"coding": [
{"system": "http://snomed.info/sct", "code": "29857009", "display": "Chest pain"},
{"system": "http://hl7.org/fhir/sid/icd-10", "code": "R07.4", "display": "Chest pain, unspecified"}
],
"text": "Chest pain, central, on exertion"
},
"subject": {"reference": "Patient/example"}
}
Two codings, one concept. Profiles decide which is required: the IPS asks for SNOMED CT where available; billing systems require the national ICD variant. See CodeableConcept and Coding and FHIR Condition explained.
How countries combine them
Countries combine SNOMED CT and ICD-10 by using the terminology for clinical records and the classification for statistics and payment. The United Kingdom mandates SNOMED CT in NHS systems with ICD-10 derived by clinical coders; the United States uses SNOMED CT for problem lists and ICD-10-CM for claims; most European countries bill with a national ICD-10 variant.
- United Kingdom: SNOMED CT is mandatory in NHS clinical systems; ICD-10 (and OPCS-4 for procedures) is used for hospital statistics and payment, derived by clinical coders.
- United States: SNOMED CT for problem lists (required by certification); ICD-10-CM for diagnoses on claims; the two are mapped at the EHR level.
- Germany, France, Italy, Spain: ICD-10 national variants (ICD-10-GM, CIM-10, ICD-9-CM in Italian discharge data, CIE-10-ES) drive reimbursement; SNOMED CT adoption is growing through national licences and EHDS-related projects.
- Nordics, Netherlands, Belgium, Australia, New Zealand: SNOMED CT members with national extensions; ICD-10 variants for reporting.
See ICD-10 vs ICD-10-CM vs ICD-10-GM for the variants.
Choosing for a new system
A new clinical system should record diagnoses in SNOMED CT when its users are clinicians in a SNOMED member country, and derive ICD-10 codes from the official map for reporting, with coders adjusting them. Recording only ICD codes suits purely billing or statistical systems; storing only an ICD code loses the detail of the SNOMED CT concept a clinician chose.
Record in SNOMED CT if your users are clinicians and your market is a SNOMED member country; derive ICD codes from the map for reporting and let coders adjust. Record in ICD only if the system exists purely for billing or statistics. Never store only an ICD code when a clinician chose a more specific SNOMED concept: the detail cannot be recovered later. And remember that in LOINC's domain (tests, measurements, documents) neither applies; see LOINC vs SNOMED CT.
Anpheros records conditions with the codes the source provided, SNOMED CT and ICD-10 together when both exist, and shows the person the plain-language name. Its symptom catalogue carries ICD-10 and SNOMED CT references for each entry so that what a person logs can be read by clinical systems. It does not assign diagnoses.
Related pages
- What is SNOMED CT?
- ICD-10 vs ICD-11
- ICD-10 vs ICD-10-CM vs ICD-10-GM
- LOINC vs SNOMED CT
- FHIR Condition explained
- CodeableConcept and Coding in FHIR
Sources
- SNOMED International: maps (SNOMED CT to ICD-10)
- NLM: SNOMED CT to ICD-10-CM map
- SNOMED International: what is SNOMED CT
- WHO: International Classification of Diseases
- WHO ICD-10 browser
- HL7 FHIR R4: using SNOMED CT with FHIR
- HL7 FHIR R4: using ICD-[X] with FHIR
Frequently asked questions
Is SNOMED CT replacing ICD?
No. They answer different questions. ICD-11 even includes a SNOMED-informed foundation layer, and the two organisations cooperate, but countries will keep a classification for statistics and a terminology for care.
Can I map ICD-10 codes to SNOMED CT automatically?
Only to a broad concept, and often ambiguously. The official map runs from SNOMED CT to ICD-10. Going the other way, you can propose candidates for a human to choose from, not a definitive concept.
Which one do patients see?
Usually neither directly: a plain-language name derived from the SNOMED description or the ICD title. Patient summaries such as the IPS carry the codes underneath so another system can read them.
Do I need a licence for SNOMED CT?
In SNOMED International member countries, use within the country is free under the national licence. Elsewhere, an affiliate licence from SNOMED International is required, with a fee that depends on the use.
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