Learning About ICD-10 Codes

Have you ever wondered how medical conditions and diseases are kept track of in many parts of the world? Well, more than 100 countries use the International Classification of Diseases (ICD), a set of standardized codes published and maintained by the World Health Organization (WHO) to keep track of diseases, causes of death, reimbursement and resource allocation.¹ But how did the ICD come about and change over the years?

Systems to classify diseases emerged as early as 1763.¹ However, the “International List of Causes of Death,” a predecessor to ICD, was not published until 1893.^1,2 This list was updated around once per decade from 1900 to 1938, until the WHO took charge in 1948, renamed the list to ICD, and expanded it to code for morbidity (disease), in addition to mortality (causes of death).² Since then, the ICD has been updated around once per decade, with ICD-10 going into effect in 1993 and ICD-11 going into effect in 2022.^2,3

In the United States, ICD, modified into ICD-10-CM by the National Center for Health Statistics (NCHS), fully transitioned from ICD-9-CM to ICD-10-CM in 2015.^1,4 ICD-10-CM codes are alphanumeric, between three and seven characters, with a decimal after the first three characters, and are arranged by body system or conditions,⁵ from A00-B99 codes for “certain infectious and parasitic diseases” to C00-D49 codes for “neoplasms,” all the way to Z00-Z99.⁶ Compared to ICD-9-CM, ICD-10-CM is more detailed and specific, and was expanded to include around 68,000 codes.⁷

For example, I69.351 is code for sequelae of cerebral infarction, hemipleFgia and hemiparesis following cerebral infarction affecting the right dominant side.⁶ One can imagine that with codes this specific in a patient chart, providers can get a more accurate idea just from ICD codes of a patient’s medical history and conditions.

Other benefits range from improved public health data collection to better coordination of care. Public health officials and researchers can more easily look at the frequency of specific medical conditions, see if diseases are spreading, and quantify how people respond to treatments, all of which can influence healthcare policy.⁸ ICD can also influence billing and reimbursement by how specific codes attached to visits are. Despite these benefits, ICD codes can be difficult to learn how to use accurately and efficiently, especially when revisions occur approximately every decade.

Overall, ICD is a set of standardized codes used by over 100 countries to keep track of morbidity and mortality. From 1893 to the present, ICD codes have been and continue to be revised, with the most recent update occurring in 2022.² ICD keeps track of healthcare utilization, reimbursements and payments, and diseases across populations to improve public health.

References

1. Hirsch JA, Nicola G, McGinty G, Liu RW, Barr RM, Chittle MD, & Manchikanti L. ICD-10: History and Context. AJNR. American Journal of Neuroradiology. 2016;37(4):596–599. https://doi.org/10.3174/ajnr.A4696
2. Moriyama IM, Loy RM, Robb-Smith AHT. History of the statistical classification of diseases and causes of death. Rosenberg HM, Hoyert DL, eds. Hyattsville, MD: National Center for Health Statistics. 2011.
3. International Statistical Classification of Diseases and Related Health Problems (ICD). World Health Organization. Accessed August 18, 2025. https://www.who.int/standards/classifications/classification-of-diseases
4. ICD-10-CM. National Center for Health Statistics. Accessed August 18, 2025. https://www.cdc.gov/nchs/icd/icd-10-cm/index.html
5. ICD-10-CM Structure. Medicare Learning Network. Accessed August 18, 2025. https://www.cms.gov/outreach-and-education/outreach/npc/downloads/slides-from-the-011414-icd10-basics-video.pdf
6. International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10)-WHO Version for 2019-covid-expanded. Accessed August 18, 2025. https://icd.who.int/browse10/2019/en
7. Transition to ICD-10. U.S. Department of Labor. Accessed August 18, 2025. https://www.dol.gov/agencies/owcp/FECA/ICD10transition
8. ICD-10 Implementation Guide for Payers. Centers for Medicare & Medicaid Services. Accessed August 18, 2025. https://www.cms.gov/files/document/icd10payerhandbook0604131pdf

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