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The Structure of ICD-11

The eleventh version of the International Classification of Diseases (ICD-11) is structured very differently than previous ICD versions. At the core of this difference is the ICD-11 Foundation component and the ability to define a linearization.

The Foundation is the underlying structure of ICD-11. It is a large data base that includes all ICD concepts, codes, and synonyms. This Foundation is the data source to define a linearization. A linearization is a subset of codes selected from the Foundation for a specific purpose, it includes a tabular list and index of the selected codes. The World Health Organization’s (WHO) ICD-11 mortality and morbidity statistics (MMS) is an example of such a linearization.

To create the ICD-11 MMS linearization, the WHO identified approximately 17,000 codes, from 80,000 concepts in the Foundation.1 In comparison, the WHO’s ICD-10 had only about 14,000 codes and no Foundation layer, so the U.S. had to create their own expanded code set (ICD-10-CM) to meet our data reporting needs. Though ICD-10-CM is based on the WHO’s ICD-10, it is a completely separate code set. Other countries modified ICD-10 to create their own expanded code set as well. With ICD-11, WHO has taken the opposite approach. The ICD-11 Foundation houses 80,000 concepts and from that, the US (and other countries) will be able to create a linearization to meet their specific needs, potentially avoiding the time and expense to create a whole new code set. Such an approach would also improve data comparability world-wide since the codes and index terms included in a linearization remain fully embedded in ICD-11.

Based on this unique structure of ICD-11, the U.S. could adopt the WHO’s ICD-11 MMS or develop their own linearization of ICD-11. This and other decisions related to ICD-11 adoption are still unknown.

1. Whitepaper 2021 | IFHIMA. (accessed 17 Feb 2021).

Are you interested in learning more about ICD-11? Watch UASI’s “Introduction to ICD-11 Codes” webinar to learn more.