International Classification of Diseases
The International Classification of Diseases is a globally used medical classification that is used in epidemiology, health management and clinical diagnosis. The ICD is maintained by the World Health Organization, which is the directing and coordinating authority for health within the United Nations System. The ICD was originally designed as a health care classification system, providing a system of diagnostic codes for classifying diseases, including nuanced classifications of a wide variety of signs, symptoms, abnormal findings, complaints, social circumstances, and external causes of injury or disease. This system is designed to map health conditions to corresponding generic categories together with specific variations; for these designated codes are assigned, each up to six characters long. Thus each major category is designed to include a set of similar diseases.
The ICD is published by the WHO and used worldwide for morbidity and mortality statistics, reimbursement systems, and automated decision support in health care. This system is designed to promote international comparability in the collection, processing, classification, and presentation of these statistics. The ICD is a major project to statistically classify all health disorders and to provide diagnostic assistance. The ICD is a core system for healthcare-related issues of the WHO Family of International Classifications.
The ICD is revised periodically and is currently in its 11th revision. The ICD-11, as it is known, was accepted by WHO's World Health Assembly on 25 May 2019 and officially came into effect on 1 January 2022. On 11 February 2022, the WHO stated that 35 countries were using the ICD-11. On 14 February 2023, the WHO reported that 64 countries were "in different stages of ICD-11 implementation". In May 2024, they stated that 50 countries were either conducting or expanding implementation pilots, and that 14 countries were actively using the ICD-11.
The ICD is part of a "family" of international classifications that complement each other, including the following classifications:
- the International Classification of Functioning, Disability and Health that focuses on the domains of functioning associated with health conditions, from both medical and social perspectives, and
- the International Classification of Health Interventions that classifies the whole range of medical, nursing, functioning and public health interventions.
In the United States and some other countries, the Diagnostic and Statistical Manual of Mental Disorders is preferred when classifying mental disorders for certain purposes.
The ICD is currently the most widely used statistical classification system for diseases in the world. In addition, some countries—including Australia, Canada, and the United States—have developed their own adaptations of ICD, with more procedure codes for classification of operative or diagnostic procedures.
Early history
In 1860, during the international statistical congress held in London, Florence Nightingale made a proposal that was to result in the development of the first model of systematic collection of hospital data. In 1893, a French physician, Jacques Bertillon, introduced the Bertillon Classification of Causes of Death at a congress of the International Statistical Institute in Chicago.A number of countries adopted Bertillon's system, which was based on the principle of distinguishing between general diseases and those localized to a particular organ or anatomical site, as used by the City of Paris for classifying deaths. Subsequent revisions represented a synthesis of English, German, and Swiss classifications, expanding from the original 44 titles to 161 titles.
In 1898, the American Public Health Association recommended that the registrars of Canada, Mexico, and the United States also adopt it. The APHA also recommended revising the system every 10 years to ensure the system remained current with medical practice advances. As a result, the first international conference to revise the International Classification of Causes of Death took place in 1900, with revisions occurring every ten years thereafter. At that time, the classification system was contained in one book, which included an Alphabetic Index as well as a Tabular List. The book was small compared with current coding texts.
The revisions that followed contained minor changes. Responsibility for ICD revisions fell to the Mixed Commission, a group composed of representatives from the ISI and the Health Organization of the League of Nations.
Versions
ICD-6
In 1948, the WHO assumed responsibility for preparing and publishing the revisions to the ICD every ten years.The ICD-6, published in 1949, was the first to be shaped to become suitable for morbidity reporting. Accordingly, the name changed from "International List of Causes of Death" to the "International Statistical Classification of Diseases, Injuries and Causes of Death". The combined code section for injuries and their associated accidents was split into two, a chapter for injuries, and a chapter for their external causes. With use for morbidity there was a need for coding mental conditions, and for the first time a section on mental disorders was added.
ICD-7
The International Conference for the Seventh Revision of the International Classification of Diseases was held in Paris under the auspices of WHO in February 1955. In accordance with a recommendation of the WHO Expert Committee on Health Statistics, this revision was limited to essential changes and amendments of errors and inconsistencies.ICD-8
The 8th Revision Conference convened by WHO met in Geneva, from 6 to 12 July 1965. This revision was more radical than the Seventh but left unchanged the basic structure of the Classification and the general philosophy of classifying diseases, whenever possible, according to their etiology rather than a particular manifestation.During the years that the Seventh and Eighth Revisions of the ICD were in force, the use of the ICD for indexing hospital medical records increased rapidly and some countries prepared national adaptations which provided the additional detail needed for this application of the ICD.
ICDA-8 (United States)
In the US, a group of consultants was asked to study the ICD-8 for its applicability to various users in the United States. This group recommended that further detail be provided for coding hospital and morbidity data. The American Hospital Association's "Advisory Committee to the Central Office on ICDA" developed the needed adaptation proposals, resulting in the publication of the International Classification of Diseases, Adapted. In 1968, the United States Public Health Service published the International Classification of Diseases, Adapted, 8th Revision for use in the United States. Beginning in 1968, ICDA-8 served as the basis for coding diagnostic data for both official morbidity and mortality statistics in the United States.ICD-9
The International Conference for the Ninth Revision of the International Statistical Classification of Diseases, Injuries, and Causes of Death, convened by WHO, met in Geneva from 30 September to 6 October 1975. In the discussions leading up to the conference, it had originally been intended that there should be little change other than updating of the classification. This was mainly because of the expense of adapting data processing systems each time the classification was revised.There had been an enormous growth of interest in the ICD and ways had to be found of responding to this, partly by modifying the classification itself and partly by introducing special coding provisions. A number of representations were made by specialist bodies which had become interested in using the ICD for their own statistics. Some subject areas in the classification were regarded as inappropriately arranged and there was considerable pressure for more detail and for adaptation of the classification to make it more relevant for the evaluation of medical care, by classifying conditions to the chapters concerned with the part of the body affected rather than to those dealing with the underlying generalized disease.
At the other end of the scale, there were representations from countries and areas where a detailed and sophisticated classification was irrelevant, but which nevertheless needed a classification based on the ICD in order to assess their progress in health care and in the control of disease. A field test with a bi-axial classification approach—one axis for anatomy, with another for etiology—showed the impracticability of such approach for routine use.
The final proposals presented to and accepted by the Conference in 1978 retained the basic structure of the ICD, although with much additional detail at the level of the four digit subcategories, and some optional five digit subdivisions. For the benefit of users not requiring such detail, care was taken to ensure that the categories at the three digit level were appropriate.
As the World Health Organization explains: "For the benefit of users wishing to produce statistics and indexes oriented towards medical care, the 9th Revision included an optional alternative method of classifying diagnostic statements, including information about both an underlying general disease and a manifestation in a particular organ or site. This system became known as the 'dagger and asterisk system' and is retained in the Tenth Revision. A number of other technical innovations were included in the Ninth Revision, aimed at increasing its flexibility for use in a variety of situations."
It was eventually replaced by ICD-10, the version currently in use by the WHO and most countries. Given the widespread expansion in the tenth revision, it is not possible to convert ICD-9 data sets directly into ICD-10 data sets, although some tools are available to help guide users.
Publication of ICD-9 without IP restrictions in a world with evolving electronic data systems led to a range of products based on ICD-9, such as MeDRA or the Read directory.