ICD-11 Guide
ICD-11: What Medical Coders Should Know
What ICD-11 is, how its structure differs from ICD-10 — stem codes, extension codes, postcoordination, the Foundation and linearizations — and its status in the U.S.
In this article
ICD-11 is the eleventh revision of the World Health Organization's International Classification of Diseases. It was adopted by the World Health Assembly in May 2019 and came into effect for WHO member-state reporting on January 1, 2022. Countries decide when and how to implement it.
How is ICD-11 structured?
ICD-11 is built as a digital classification rather than a printed book.
- The Foundation Component is the underlying semantic network of all ICD-11 entities — diseases, disorders, injuries, causes and more — with definitions and relationships.
- Linearizations are subsets of the Foundation organized for a purpose. The main one for statistics and coding is the Mortality and Morbidity Statistics (MMS) linearization.
- Chapters cover body systems and other content, including new chapters such as sleep-wake disorders, conditions related to sexual health, traditional medicine conditions, a functioning section, and extension codes.
How do ICD-11 codes look?
MMS stem codes are alphanumeric, typically four to six characters, with a dot after the fourth character for more detailed codes (for example, 1A00 for cholera or BA00 for essential hypertension). The second character is always a letter, which helps distinguish ICD-11 codes from ICD-10 codes.
What are extension codes and postcoordination?
Extension codes add detail — such as severity, laterality, anatomy, histopathology, or the substance involved — that is not built into the stem code. Combining a stem code with extension codes (or with other stem codes) to describe a clinical picture is called postcoordination. Combined codes are written as a cluster, linked with & (adding extension detail) or / (linking stem codes).
This is a major shift from ICD-10-CM, where specificity is mostly precoordinated — built into long, specific codes.
ICD-11 vs ICD-10-CM at a glance
| Feature | ICD-10-CM (U.S.) | ICD-11 MMS (WHO) |
|---|---|---|
| Owner | NCHS/CDC (U.S. clinical modification of WHO ICD-10) | WHO |
| Specificity | Mostly precoordinated in long codes | Stem codes plus postcoordination with extension codes |
| Format | Letter first, up to 7 characters | Alphanumeric stems, second character a letter |
| Primary format | Book and files | Digital: browser, coding tool and API |
| U.S. use | Required HIPAA code set for diagnoses | Not adopted for U.S. morbidity coding |
Why should U.S. coders learn ICD-11 now?
- Global work: coders supporting international clients or research encounter ICD-11 data.
- Concept knowledge: ICD-11's terminology and structure influence classification thinking and digital health standards.
- Future readiness: any U.S. transition would likely involve a long planning period; early familiarity helps.
How to explore ICD-11
The WHO ICD-11 Browser and Coding Tool are freely available online. Search a diagnosis, look at its stem code, and then explore which extension codes can be added — it is the fastest way to understand postcoordination.
Related reading
Source
World Health Organization — ICD-11
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Additional references
Medical coding trainer and author with more than 22 years of experience, an AHIMA Approved ICD-10 Trainer, and the managing editor of MedicalCodingUpdates.com.
Educational information only. Verify coding decisions against current official guidelines, code sets and applicable payer policies. Read the full disclaimer.
