ICD

The International Classification of Diseases is the World Health Organization's standard for classifying diseases, injuries and causes of death. It is the backbone of mortality and morbidity statistics worldwide, and in most countries it is also what claims and reporting run on.

Classification, not terminology

This distinction decides which standard you should use:

	ICD	SNOMED CT
Purpose	Statistics, reporting, billing	Clinical documentation
Structure	Mutually exclusive categories	Poly-hierarchy of concepts
Granularity	Aggregation-oriented	Highly detailed
Rule	Every case fits exactly one category	Concepts combine freely

A classification exists so that cases can be counted. Every case must land in one and only one bucket, which is why ICD has residual categories such as "other specified" and "unspecified".

ICD-10

Structure: a letter followed by digits, grouped into chapters.

A00–B99   Certain infectious and parasitic diseases
E10–E14   Diabetes mellitus
I20–I25   Ischaemic heart diseases
O00–O99   Pregnancy, childbirth and the puerperium

Example: E11.9 — Type 2 diabetes mellitus without complications.

ICD-10 has national modifications (ICD-10-CM in the US, ICD-10-AM in Australia), so "ICD-10" alone does not fully specify a coding system.

ICD-11

Adopted by the World Health Assembly in 2019 and in force since January 2022. Substantive changes:

Digital-first. A maintained API and terminology service rather than a book that happens to be published electronically.
Foundation and linearizations. One underlying multi-dimensional foundation, from which purpose-specific linearizations (mortality, morbidity, primary care) are derived.
Post-coordination. Clusters of codes can express detail that ICD-10 had to leave out.
SNOMED CT mapping maintained as part of the release.
New and reorganized content, including traditional medicine and a restructured chapter on mental, behavioural and neurodevelopmental disorders.

Using ICD in FHIR

ICD codes travel in a CodeableConcept:

{
  "code": {
    "coding": [{
      "system": "http://hl7.org/fhir/sid/icd-10",
      "code": "E11.9",
      "display": "Type 2 diabetes mellitus without complications"
    }],
    "text": "Type 2 diabetes"
  }
}

For ICD-11 the system URI is http://id.who.int/icd/release/11/mms.

Implementation notes

Coding quality is a training problem, not a software problem. Unspecified codes dominate when clinicians code without support.
Migration between ICD-10 and ICD-11 is not one-to-one. Time series across a transition need explicit mapping and documented breaks.
Do not use ICD for clinical decision support. Its categories are built for counting, not for reasoning; use SNOMED CT for that.

References

WHO ICD-11 browser and API — https://icd.who.int/
SNOMED CT — clinical terminology
LOINC — laboratory observations

Classification, not terminology​

ICD-10​

ICD-11​

Using ICD in FHIR​

Implementation notes​

References​