# ICD-10 Diagnosis Code Key: `identity.medical.icd10` International Classification of Diseases, 10th Revision code used to classify diagnoses and procedures. Format: letter + 2 digits + optional dot + up to 4 alphanumeric characters. Page: Samples: `E11.9`, `J18.9`, `M54.5`, `I10`, `Z00.00` ## Background ### Origin ICD-10 is the tenth revision of the International Classification of Diseases.[[1]](https://en.wikipedia.org/wiki/ICD-10) The World Health Organization owns and publishes ICD-10.[[2]](https://www.cdc.gov/nchs/icd/icd-10-cm/index.html) Work on ICD-10 began in 1983.[[1]](https://en.wikipedia.org/wiki/ICD-10) The CDC's National Center for Health Statistics is responsible for the clinical modification of ICD-10 for use in the United States, ICD-10-CM.[[2]](https://www.cdc.gov/nchs/icd/icd-10-cm/index.html) The World Health Organization authorised NCHS to develop ICD-10-CM.[[2]](https://www.cdc.gov/nchs/icd/icd-10-cm/index.html) All modifications to ICD-10 must conform to WHO's established ICD structure and conventions.[[2]](https://www.cdc.gov/nchs/icd/icd-10-cm/index.html) ICD-10-CM is a morbidity classification for classifying diagnoses and reason for visits in all health care settings.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) On October 1, 2015, the US health care industry moved from ICD-9 to ICD-10 codes.[[4]](https://www.cms.gov/medicare/coding-billing/icd-10-codes) ### Use ICD-10-CM diagnosis codes are composed of codes with 3, 4, 5, 6 or 7 characters.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) A three-character code is to be used only if it is not further subdivided.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) A code is invalid if it has not been coded to the full number of characters required for that code.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) ICD-10-CM uses the letter X as a placeholder at certain codes to allow for future expansion.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) If a code that needs a 7th character is not 6 characters long, the placeholder X fills the empty positions.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) Because a valid code's length depends on where it sits in the classification, check values against the official code list for the right year rather than trusting a pattern alone. (Meridian's view) The ICD-10-CM diagnosis codes have been adopted under HIPAA for all healthcare settings.[[3]](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) The WHO base classification allows more than 14,000 codes.[[1]](https://en.wikipedia.org/wiki/ICD-10) ICD-10-CM has over 70,000 codes.[[1]](https://en.wikipedia.org/wiki/ICD-10) A column of ICD-10 values may come from the WHO base classification or from a national modification such as ICD-10-CM, so find out which before you look codes up. (Meridian's view) Keep the codes as text, so that nothing trims a trailing zero from a value like Z00.00. (Meridian's view) ### Where you'll meet it CDC WONDER's Underlying Cause of Death database for 1999-2020 breaks deaths down by cause of death as a 4-digit ICD-10 code or group of codes.[[5]](https://wonder.cdc.gov/ucd-icd10.html) ## Sources 1. [ICD-10 - Wikipedia](https://en.wikipedia.org/wiki/ICD-10) — secondary source 2. [ICD-10-CM \| Classification of Diseases, Functioning, and Disability \| CDC](https://www.cdc.gov/nchs/icd/icd-10-cm/index.html) — documentation 3. [ICD-10-CM Official Guidelines for Coding and Reporting FY 2026](https://ftp.cdc.gov/pub/health_statistics/nchs/publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf) — documentation 4. [ICD-10 \| CMS](https://www.cms.gov/medicare/coding-billing/icd-10-codes) — documentation 5. [Underlying Cause of Death, 1999-2020 Request](https://wonder.cdc.gov/ucd-icd10.html) — dataset