International Classification of Diseases (ICD)

Source: Park's Textbook of Preventive and Social Medicine, pp. 60-61

Background: Need for Disease Classification

There is wide variation among countries in the criteria and standards adopted for the diagnosis and notification of diseases, making it difficult to compare national statistics. A system of classification was needed whereby diseases could be grouped according to certain common characteristics to facilitate the statistical study of disease phenomena.

Historical development of classification criteria:

John Graunt in the 17th century arranged diseases alphabetically in his "Bills of Mortality"
Later, a more scientific approach classified diseases according to:
- (a) The part of the body affected
- (b) The aetiologic agent
- (c) The kind of morbid change produced by the disease
- (d) The kind of disturbance of function produced by the disease or injury

There are many axes of classification, and the particular axis selected depends on the interest of the investigator.

International Classification of Diseases (ICD)

All the above criteria formed the basis of the ICD, produced by the WHO and accepted in 1940 for national and international use.

Key historical milestones:

Revised approximately every 10 years since inception
6th Revision (1948): Scope was expanded to cover morbidity from illness and injury (earlier it only covered mortality)
10th Revision (ICD-10): Came into effect on January 1, 1993
ICD-11: Adopted by the World Health Assembly in 2019, came into effect January 1, 2022

Purpose and Functions of ICD

ICD is the foundation for the identification of health trends and statistics globally and is the international standard for reporting diseases and health conditions. It is a diagnostic classification standard for all clinical and research purposes.

ICD defines the universe of diseases, disorders, injuries, and other related health conditions, listed in a comprehensive, hierarchical fashion that allows for:

Easy storage, retrieval, and analysis of health information for evidence-based decision-making
Sharing and comparing health information between hospitals, regions, settings, and countries
Data comparison in the same location across different time periods

ICD-11 - Key Features

ICD-11 has been updated for the 21st century to reflect significant progress in science and medicine over the past 30 years. It is designed for use with digital health applications.

Scale:

Comprises over 55,000 entities
Includes 120,000 clinical terms (can code millions of terms)
Thousands of new categories and updated classification schemes

What ICD-11 covers (beyond just diseases):

Disorders, injuries, external causes
Signs and symptoms
Substances, medicaments
Anatomy, devices, histopathology, severity

Notable changes in ICD-11:

New chapter on sexual health - gender incongruence moved here (no longer a mental health condition), reducing stigma
New chapter on traditional medicine (commonly used across many countries)
Stroke is now listed as a neurological disorder, not as a circulatory system disorder
HIV reclassified to recognize advances in therapy - treated as a chronic condition
Allergy is coded under diseases of the immune system
ADHD - symptoms no longer have to occur within a fixed age range
Better reporting of antimicrobial resistance, aligned with the Global Antimicrobial Resistance Surveillance System

ICD-11 Chapters (Complete List)

#	Chapter
1	Certain infectious and parasitic diseases
2	Neoplasms
3	Diseases of blood and blood-forming organs
4	Disorders of the immune system
5	Endocrine, nutritional and metabolic diseases
6	Mental, behavioural or neurodevelopmental disorders
7	Sleep-wake disorders
8	Diseases of the visual system
9	Diseases of the nervous system
10	Diseases of the ear and mastoid process
11	Diseases of the circulatory system
12	Diseases of the respiratory system
13	Diseases of the digestive system
14	Diseases of the skin
15	Diseases of the musculoskeletal system and connective tissue
16	Diseases of the genitourinary system
17	Conditions related to sexual health
18	Pregnancy, childbirth and the puerperium
19	Certain conditions originating in the perinatal and neonatal period
20	Developmental anomalies
21	Symptoms, signs or clinical findings, not elsewhere classified
22	Injury, poisoning or certain other consequences of external causes
23	External causes of morbidity and mortality
24	Factors influencing health status or contact with health services
25	Codes for special purposes
26	Supplementary chapter - Traditional Medicine Conditions (Module I)
V	Supplementary section for functioning assessment
X	Extension codes

Linkages with Other Classifications

The ICD-11 incorporates links with the following through the ICD-11 Foundation:

ICD-O - International Classification of Diseases for Oncology
ICD-10 (ICECI) - International Classification of External Causes of Injury
ICF - International Classification of Functioning, Disability and Health
ICPC - International Classification of Primary Care
Other terminologies: OrphaNet and SNOMED-CT

International Classification of Functioning, Disability and Health (ICF)

The ICF (WHO, 2001) is a framework for organizing and documenting information on functioning and disability. It conceptualizes functioning as a "dynamic interaction between a person's health condition, environmental factors and personal factors."

Provides a standard language and conceptual basis for the definition and measurement of disability
Integrates both the medical model and the social model of disability as a bio-psycho-social synthesis
Recognizes the role of environmental factors in the creation of disability

Park's Textbook of Preventive and Social Medicine, pp. 60-61

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