The main structural innovation of ICD–11 is that it is built on a Foundation Component from which the Tabular lists (such as the classification for morbidity and mortality statistics) are derived.
The codes of the ICD–11 are alphanumeric and cover the range from 1A00.00 to ZZ9Z.ZZ. These are referred to as stem codes. The structure of stem codes is described below:
For example, 1A00 is a code in Chapter 01, and BA00 is a code in Chapter 11.
For example: ED1E.EE
The terminal letter Y is reserved for the residual category ‘other specified’ and the terminal letter ‘Z’ is reserved for the residual category ‘unspecified’. For the chapters that have more than 240 blocks, ‘F’ (‘other specified’) and ‘G’ (‘unspecified’) are also used to indicate residual categories (due to limitations in the coding space).
Throughout the Reference Guide a dash indicates that more detailed codes exist, and should be used as appropriate. For example
All entities have a Uniform Resource Identifier (URI) which is a string of characters that uniquely identifies a particular ICD-11 entity. Each entity has a specific place in the hierarchy of groups and categories.
Higher level entities in ICD-11 (called ‘blocks’) may be used for reporting aggregated statistics. However, blocks do not have category codes as they are not supposed to be used in coding. Blocks have their own URIs (e.g. the URI for Neoplasms is http://id.who.int/icd/entity/1630407678). Blocks may also be referred to by block IDs. The code structure for block IDs are 11 characters long (e.g. “BlockL1-1A0”).
Codes in the Tabular list of ICD-11 that can be used alone. Stem codes may be entities or groupings of high relevance in any of the use cases, or clinical conditions that should always be described as one single category. Stem codes are designed to ensure that in use cases that require only one code per case, meaningful information is collected.
The extension codes are comprised of groups of codes e.g. anatomy, agent, histopathology and other aspects that may be used to add detail to a stem code. Extension codes are not to be used alone in the context of statistical classification but must be added to a stem code. Extension codes may be used in another context, e.g. for device documentation. Not all extension codes can be used with every stem code. Refer to 2.9.
Postcoordination is a notable new feature in ICD-11 that creates the ability to link core diagnostic concepts (i.e. stem+stem code concepts) when desired, and/or to add clinical concepts captured in extension codes to primary stem code concepts. The linked diagnostic concepts are called a cluster. It should be emphasised that the postcoordination ability inherent in ICD-11 is one of the significant changes compared with ICD-10. A cluster describes one clinical concept as described by the health care practitioner. Postcoordination is by matter of principle permitted with all axes, as long as at least one stem code is used. API, Browser and coding tool facilitate postcoordination in certain areas. Other areas of postcoordination have to be coded individually, but may be supported by future updates of ICD-11, where need arises.
ICD–11 categories have short descriptions, plus long descriptions labelled ‘additional information’. The short description is a maximum of 100 words relating to the entity that states things that are always true about a disease or condition which are necessary to understand the scope of the rubric. It appears in the Tabular list of the classification. The long ‘additional information’ is the full description, without length restriction.
Special tabulation lists (refer to section 2.25) continue to exist in ICD-11, but there are three additional lists - the Startup Mortality List (SMoL), the list for verbal autopsy, and the list for infectious diseases by agent. Specialty linearisations allow the representation of content from the perspective of a specialty, such as dermatology or neurology, through the creation of subsets, and through the precoordination of more detail, if desired.