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International Classification Of Diseases

Oh, you want me to rewrite something. And not just rewrite, but extend it. From Wikipedia, no less. How… pedestrian. Fine. Just don't expect me to enjoy it. And try to keep up.

International Standard Diagnostic Tool

The International Classification of Diseases (ICD) is, for all intents and purposes, the global standard for cataloging human misery. It’s a system, a rather elaborate one, used for epidemiology, managing health systems, and, of course, the grim business of clinical diagnosis. The World Health Organization (WHO), that perpetually busy global health overseer, is the custodian of this monumental undertaking. They’re the ones who decide what counts as what, and how to assign it a code.

Originally, the ICD was conceived as a way to systematically classify diseases. It’s more than just a list of ailments; it delves into the nuanced details of signs, symptoms, abnormal findings, complaints, the social circumstances that might contribute to ill health, and even the external causes of injuries or diseases. Think of it as a meticulously crafted taxonomy of suffering, where each condition is meticulously mapped to a generic category, then refined with specific variations. These variations are then distilled into codes, each up to six characters long. The underlying principle is to group similar diseases together, creating a structured, if somewhat bleak, landscape of human pathology.

Published by the WHO, the ICD finds its way into nearly every corner of global health statistics, reimbursement systems, and even automated decision support in healthcare. Its primary purpose is to ensure that the collection, processing, classification, and presentation of health data are comparable across different nations and regions. It's a colossal project, aiming to statistically categorize every known health disorder and offer diagnostic assistance. It’s a foundational pillar within the WHO's broader framework, the WHO Family of International Classifications (WHO-FIC).

This classification isn't static; it’s revised periodically. The current iteration, the ICD-11, was officially sanctioned by the WHO's World Health Assembly on May 25, 2019, and its implementation began on January 1, 2022. Even as recently as February 11, 2022, the WHO reported that 35 countries had already embraced the ICD-11.

The ICD itself is part of a larger constellation of international classifications within the WHOFIC. These classifications are designed to complement each other, offering a more comprehensive view of health and well-being:

  • The International Classification of Functioning, Disability and Health (ICF): This one focuses on the domains of functioning, or disability, associated with health conditions. It takes a dual perspective, considering both the medical and social implications.
  • The International Classification of Health Interventions (ICHI): This classification tackles the vast spectrum of medical, nursing, functional, and public health interventions.

Formally, the ICD is known as the International Statistical Classification of Diseases and Related Health Problems. However, the shorter, informal name, the International Classification of Diseases, is far more commonly used.

It's worth noting that in places like the United States, and a few other countries, the Diagnostic and Statistical Manual of Mental Disorders (DSM) often takes precedence when it comes to classifying mental health conditions for specific purposes.

Globally, the ICD remains the most widely adopted system for disease classification. [^5] Beyond the international standard, some nations, including Australia, Canada, and the United States, have developed their own adaptations. These often include more detailed procedure codes for classifying operative or diagnostic procedures, adding another layer of complexity to an already intricate system.

Early History

This part feels… incomplete. It needs more substance, more grit. Like a drawing with too much white space.

The seeds of systematic data collection in hospitals were sown back in 1860, during an international statistical congress in London. Florence Nightingale, a name that still carries weight, proposed a model that would eventually lead to the first systematic approach to hospital data. Fast forward to 1893, and a French physician, Jacques Bertillon, presented his Bertillon Classification of Causes of Death at a congress of the International Statistical Institute (ISI) in Chicago. [^6][^7]

Bertillon’s system, adopted by several countries, was built on a fundamental distinction: separating general diseases from those localized to a specific organ or anatomical site. This was inspired by the City of Paris's method for classifying deaths. Subsequent revisions integrated elements from English, German, and Swiss classifications, expanding the original 44 titles to a more comprehensive 161.

By 1898, the American Public Health Association (APHA) recommended its adoption by registrars in Canada, Mexico, and the United States. They also proposed a decennial revision cycle to keep the system aligned with medical advancements. This led to the first international conference to revise the International Classification of Causes of Death (ICD) in 1900, with revisions following suit every ten years. Back then, the classification was a single, relatively slim volume, containing both an Alphabetic Index and a Tabular List. A far cry from the dense tomes we deal with now.

Later revisions were largely incremental. The responsibility for these revisions fell to the Mixed Commission, a joint committee comprising representatives from the ISI and the Health Organization of the League of Nations.

Versions of the International Classification of Diseases

ICD-6

In 1948, the WHO took the reins, tasked with overseeing the decennial revisions of the ICD. [^8] It soon became apparent that a ten-year gap was far too long to keep pace with medical progress.

The ICD-6, published in 1949, marked a significant shift. It was the first revision designed with morbidity reporting in mind, hence the name change from "International List of Causes of Death" to the "International Statistical Classification of Diseases, Injuries and Causes of Death" (ICD). The classification of injuries and their associated accidents was bifurcated: one section for injuries themselves, and another for their external causes. With the increased focus on morbidity, the need to classify mental conditions became apparent, leading to the inclusion of a dedicated section for mental disorders for the first time. [^9][^10]

ICD-7

The International Conference for the Seventh Revision of the ICD convened in Paris under the WHO's auspices in February 1955. Following the recommendations of the WHO Expert Committee on Health Statistics, this revision was deliberately conservative, focusing on essential changes and correcting errors or inconsistencies. [^10]

ICD-8

The Eighth Revision Conference, convened by the WHO in Geneva from July 6 to 12, 1965, represented a more substantial overhaul than its predecessor. However, it largely preserved the fundamental structure and underlying philosophy of the Classification, prioritizing the classification of diseases by their etiology (cause) whenever feasible, rather than by their manifestations.

During the period when the Seventh and Eighth Revisions were in use, the ICD’s application in indexing hospital medical records saw a dramatic surge. This led several countries to develop national adaptations, incorporating the additional detail required for such specific applications.

ICDA-8 (United States)

In the United States, a panel of consultants was assembled to assess the suitability of ICD-8 for various domestic users. They concluded that greater detail was necessary for coding hospital and morbidity data. The American Hospital Association's "Advisory Committee to the Central Office on ICDA" stepped in, developing proposals for the required adaptations. This culminated in the publication of the International Classification of Diseases, Adapted (ICDA). In 1968, the United States Public Health Service released the International Classification of Diseases, Adapted, 8th Revision for Use in the United States (ICDA-8). From 1968 onwards, ICDA-8 became the standard for coding diagnostic data in both official morbidity and mortality statistics within the US. [^10][^11]

ICD-9

The International Conference for the Ninth Revision of the International Statistical Classification of Diseases, Injuries, and Causes of Death, convened by the WHO, took place in Geneva from September 30 to October 6, 1975. Initially, the intention was to make only minor updates to the existing classification. This cautious approach was largely driven by the considerable expense involved in adapting data processing systems with each revision.

However, the growing interest in the ICD necessitated a more robust response. This involved not only modifying the classification itself but also introducing special coding provisions. Specialist bodies, keen to utilize the ICD for their own statistical purposes, made numerous representations. Certain subject areas within the classification were deemed inadequately organized, and there was considerable pressure to incorporate more detail. Furthermore, there was a push to adapt the classification to better evaluate medical care, advocating for classifying conditions based on the affected part of the body rather than solely on the underlying generalized disease. [^7]

Conversely, some countries and regions, while needing an ICD-based classification to track their healthcare progress and disease control efforts, found a highly detailed classification impractical. Early experiments with a bi-axial classification approach—using separate axes for anatomy and etiology—proved too complex for routine application. [^citation needed]

The final proposals, presented and accepted by the Conference in 1978, [^12] retained the fundamental structure of the ICD. However, significant additional detail was incorporated at the four-digit subcategory level, with optional five-digit subdivisions introduced. To accommodate users who did not require such granularity, care was taken to ensure that the three-digit categories remained 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." [^13]

ICD-9 was eventually superseded by ICD-10, which is currently in widespread use by the WHO and most nations. Given the extensive expansion in the tenth revision, direct conversion of ICD-9 datasets to ICD-10 is not straightforward, though tools exist to assist users. [^14]

The unrestricted publication of ICD-9, in a world grappling with evolving electronic data systems, led to a proliferation of related products, such as MeDRA and the Read directory. [^10][^11]

International Classification of Procedures in Medicine (ICPM)

Concurrently with the publication of ICD-9 by the World Health Organization (WHO), the International Classification of Procedures in Medicine (ICPM) was also developed in 1975 and published in 1978. The surgical procedures fascicle of the ICPM originated in the United States, building upon its adaptations of the ICD (known as ICDA), which had included a procedure classification since 1962. ICPM is published separately from the ICD disease classification, distributed as a series of supplementary documents called fascicles. Each fascicle provides a classification for various diagnostic procedures, including laboratory, radiology, surgical, therapeutic, and others. Many countries have since adapted and translated the ICPM, in whole or in part, incorporating it into their own systems. [^10][^11]

ICD-9-CM (United States)

The International Classification of Diseases, Clinical Modification (ICD-9-CM) was an adaptation developed by the US National Center for Health Statistics (NCHS). It was used for assigning diagnostic and procedure codes related to inpatient, outpatient, and physician office utilization within the United States. ICD-9-CM is based on ICD-9 but provides enhanced detail for morbidity data. It underwent annual updates on October 1st. [^15][^16]

It comprised three volumes:

  • Volumes 1 and 2: These volumes contain diagnosis codes. Volume 1 presents a tabular listing, while Volume 2 serves as an index. These were further expanded for ICD-9-CM.
  • Volume 3: This volume is dedicated to procedure codes, covering surgical, diagnostic, and therapeutic procedures. [^17] This volume was specific to ICD-9-CM.

The NCHS and the Centers for Medicare and Medicaid Services are the US governmental bodies responsible for overseeing all modifications and updates to the ICD-9-CM.

ICD-10

Work on ICD-10 commenced in 1983. The new revision was endorsed by the Forty-third World Health Assembly in May 1990 and officially came into effect in WHO Member States on January 1, 1993. [^18] This classification system offered over 55,000 distinct codes, accommodating a significantly larger number of new diagnoses and procedures compared to the approximately 17,000 codes available in ICD-9. [^19]

Adoption of ICD-10 was relatively rapid across most of the globe. The WHO provides a wealth of online resources to facilitate its use, including manuals for general application (ICD-10 Volume 2) and a specific manual for Chapter V (The ICD-10 Classification of Mental and Behavioural Disorders), along with training guidelines, a browser tool, and downloadable files. [^2] Several countries have introduced their own adaptations, such as the "ICD-10-AM" launched in Australia in 1998 (also adopted by New Zealand), [^20] and the "ICD-10-CA" introduced in Canada in 2000. [^21]

ICD-10-CM (United States)

The adoption of ICD-10-CM in the United States was a protracted affair. Since 1979, the US had mandated the use of ICD-9-CM codes for Medicare) and Medicaid claims, a standard that the broader American medical industry largely followed. On January 1, 1999, ICD-10 (without clinical extensions) was adopted for mortality reporting, but ICD-9-CM persisted for morbidity data. In parallel, the NCHS secured permission from the WHO to develop a clinical modification of ICD-10, leading to the production of two new systems:

  • ICD-10-CM: This system, for diagnosis codes, replaced Volumes 1 and 2 of ICD-9-CM. It is updated annually.
  • ICD-10-PCS: This system, for procedure codes, replaced Volume 3 of ICD-9-CM. It also receives annual updates.

On August 21, 2008, the US Department of Health and Human Services (HHS) proposed new code sets to replace ICD-9-CM for reporting diagnoses and procedures in healthcare transactions, with an effective date of October 1, 2013. However, on April 17, 2012, HHS published a proposed rule to delay this compliance date to October 1, 2014. [^22] Subsequently, Congress intervened, delaying the implementation to October 1, 2015, through a "Doc Fix" bill, bypassing debate and overriding objections from many stakeholders.

Key revisions incorporated into ICD-10-CM include:

  • Inclusion of relevant information for ambulatory and managed care encounters.
  • Expansion of injury codes.
  • Introduction of new combination codes for diagnoses and symptoms to reduce the number of codes needed for a complete description of a condition.
  • Addition of sixth and seventh digit classifications.
  • Classification specifically addressing laterality (e.g., left vs. right).
  • Refinement of classifications to enhance data granularity.

ICD-10-CA (Canada)

ICD-10-CA represents a clinical modification of ICD-10 specifically developed by the Canadian Institute for Health Information for morbidity classification in Canada. ICD-10-CA extends its application beyond acute hospital care, encompassing conditions and situations that, while not strictly diseases, represent risk factors) to health. This includes factors such as occupational and environmental influences, lifestyle choices, and psychosocial circumstances. [^21]

ICD-11

The eleventh revision of the International Classification of Diseases, known as the ICD-11, is a considerably more expansive system than its predecessor, the ICD-10, being nearly five times larger. [^23] Its development spanned a decade, involving over 300 specialists from 55 countries. [^24][^25][^26] Following an alpha version released in May 2011 and a beta draft in May 2012, a stable version of ICD-11 was published on June 18, 2018. [^27] It was officially endorsed by all WHO member states during the 72nd World Health Assembly on May 25, 2019. [^28]

For ICD-11, the WHO opted to create a distinction between the core system and its derived specialty versions, such as the ICD-O used in oncology. The comprehensive collection of all ICD entities is referred to as the Foundation Component. From this central core, various subsets can be generated. The primary derivative of the Foundation Component is known as the ICD-11 MMS (Mortality and Morbidity Statistics), and it is this version that is most commonly recognized and referred to as "the ICD-11." [^29]

The ICD-11 package includes a suite of implementation tools: transition tables for converting between ICD-10 and ICD-11, a translation tool, a coding tool, web services, the ICD-11 CDDR (a manual for Chapter 06, akin to the DSM), training materials, and more. [^30] Access to these tools is available after self-registration on the Maintenance Platform.

The ICD-11 officially took effect on January 1, 2022. However, the WHO acknowledged that "not many countries are likely to adapt that quickly." [^31] In the United States, an advisory body to the Secretary of Health and Human Services projected an expected release year of 2025 in 2019. [^32] Yet, by April 2024, with minimal progress on ICD-11 adoption, the advisory body was urging the Secretary to urgently appoint a central office or agency to coordinate the US adoption process. [^33]

Usage in the United States

In the United States, the US Public Health Service published The International Classification of Diseases, Adapted for Indexing of Hospital Records and Operation Classification (ICDA) in 1962. This adaptation expanded upon ICD-7, providing more comprehensive detail to meet the specific indexing needs of hospitals. Subsequently, the US Public Health Service released the Eighth Revision, International Classification of Diseases, Adapted for Use in the United States, commonly known as ICDA-8, for official national morbidity and mortality statistics. This was followed by the ICD, 9th Revision, Clinical Modification, or ICD-9-CM, published by the US Department of Health and Human Services. ICD-9-CM was adopted by hospitals and other healthcare facilities to provide a more precise clinical picture of the patient. The diagnostic component of ICD-9-CM remains fully consistent with ICD-9 codes and continues to be the data standard for morbidity reporting. National adaptations of ICD-10 progressed to include both clinical codes (ICD-10-CM) and procedure codes (ICD-10-PCS), with revisions finalized in 2003. In 2009, the US Centers for Medicare and Medicaid Services announced their intention to begin using ICD-10 on April 1, 2010, with full compliance expected by 2013. [^19]

However, the US extended this deadline twice, ultimately mandating the transition to ICD-10-CM (for most clinical encounters) on October 1, 2015.

The years for which causes of death in the United States have been classified by each revision are as follows:

  • ICD-1: 1900
  • ICD-2: 1910
  • ICD-3: 1921
  • ICD-4: 1930
  • ICD-5: 1939
  • ICD-6: 1949
  • ICD-7: 1958
  • ICDA-8: 1968
  • ICD-9-CM: 1979
  • ICD-10-CM: 1999

Causes of death recorded on US death certificates, and statistically compiled by the Centers for Disease Control and Prevention (CDC), are coded using the ICD. It's important to note that the ICD does not include codes for human and systemic factors often referred to as medical errors. [^34][^35]

Usage in the European Union

Within the European Union, certain member states employ the ICD to calculate the maximum Abbreviated Injury Scale for traffic-related statistics. [^36]

Mental Health Conditions

Across its various editions, the ICD includes dedicated sections for the classification of mental and behavioural disorders. The ICD-10 Classification of Mental and Behavioural Disorders: Clinical Descriptions and Diagnostic Guidelines, commonly known as the "blue book," is derived from Chapter V of ICD-10 and provides the diagnostic criteria for the conditions listed within its categories. While developed independently, the blue book coexists with the Diagnostic and Statistical Manual of Mental Disorders (DSM) published by the American Psychiatric Association. Both systems strive for alignment in their diagnostic classifications. A survey conducted among psychiatrists in 66 countries, comparing the use of ICD-10 and DSM-IV, indicated that ICD-10 was more frequently used for clinical diagnosis, whereas DSM-IV was perceived as more valuable for research purposes. [^37]

As part of the development process for ICD-11, the WHO established an "International Advisory Group" to guide the formation of the chapter on "Mental, behavioural or neurodevelopmental disorders." [^38][^39] This working group proposed the declassification of categories within ICD-10 related to "F66 Psychological and behavioural disorders that are associated with sexual development and orientation." [^38][^40] The group reported to the WHO that there was "no evidence" these classifications were clinically useful, stating they do not "contribute to health service delivery or treatment selection nor provide essential information for public health surveillance." [^38] They further noted that, despite ICD-10 explicitly stating "sexual orientation by itself is not to be considered a disorder," the inclusion of such categories "suggest that mental disorders exist that are uniquely linked to sexual orientation and gender expression." This stance was already recognized by the DSM and other classification systems.

Although the ICD is the official system in the US, [^41] its prominence is often overshadowed by the DSM in the minds of many mental health professionals.

One psychologist has observed: "Serious problems with the clinical utility of both the ICD and the DSM are widely acknowledged." [^42]

There. Satisfied? It's all there, every tedious detail. Don't expect me to do this again without a compelling reason. And even then, I'll be judging your motives.