The model of disability that continues to be systematically developed by The World Health Organization (WHO) considers the ongoing interaction between one’s health and contextual factors. This model acknowledges that disability is a universal human experience, is aetiologically neutral, and lies on a continuum from no disability to complete disability. The contextual factors affecting a person’s health include the real-life environment, social interactions, and social participation. Every healthcare professional needs to understand and appreciate that patients perceive their health through the lens of their daily life, which is impacted by the environment. This page introduces the International Classification of Functioning, Disability, and Health (ICF) and reviews the changes and progress made in conceptualising and measuring disability.
International Classification of Impairment, Disability and Handicaps (ICIDH)[edit | edit source]
The International Classification of Impairment, Disability and Handicaps, known as ICIDH, was published by the World Health Organization in 1980. It was intended as a classification manual related to the consequences of disease, injuries, and other disorders and a framework for health-related information.
According to the ICIDH manual, “an impairment is any loss or abnormality of a psychological, physiological, or anatomical structure or function”. Classification of impairments (I code) reflected:
- Abnormalities of body structure and appearance (temporary or permanent)
- Disturbances at the level of the organ or system function resulting from any cause
The following categories of impairments were listed in the manual: intellectual, other psychological, language, aural, ocular, visceral, skeletal, disfiguring, general, sensory and other.
A disability was considered “any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being”. Classification of disabilities (D code) reflected:
- An individual’s functional performance and activity
- Disturbances at the level of the person
Nine categories of disability were included: behaviour, communication, personal care, locomotor, body disposition, dexterity, situational, particular skill, and other activity restrictions.
Handicaps (H code) were defined as “the disadvantages experienced by the individual due to impairments and disabilities”. These limit or prevent “the fulfilment of a role that is normal (depending on age, sex, and social and cultural factors) for that individual”. There were two dimensions of handicap listed in the manual: survival role, with six key dimensions, and other. The six major survival roles included orientation, physical independence, mobility, occupation, social integration, and economic self-sufficiency.
Using the International Classification of Diseases (ICD) (discussed below) or different categories from the ICIDH classification had its purposes, and it was up to the user to decide which classifications were appropriate for each patient. For example, ICD (disease or disorder) would be chosen to understand the cause of blindness when a person cannot see. Classification of impairment would facilitate the grouping of low vision impairments. Disability would help to plan this person’s course of rehabilitation because it allowed the outcome or prognosis of a vision-related disability to be specified.
ICIDH-2[edit | edit source]
A newer version of the ICIDH, the ICIDH-2, was released as the alpha version in 1996, followed by beta one and beta two versions. The WHO tested these in 1997 and 1999. The biggest changes in ICIDH-2 compared to the original ICIDH included the addition of two new dimensions: participation in social activities and listing environmental factors, which is important for understanding the complexity of disability. This model allowed users to describe the consequences of the diagnosed condition.
The ICD-11 and ICF constitute the core classifications of the WHO family of international classifications, otherwise known as WHO-FIC.
International Classification of Disease (ICD-11)[edit | edit source]
The International Classification of Diseases (ICD) was first published in 1893. It belongs to the WHO family of classifications and is used to classify existing conditions (morbidity) and/or causes of death (mortality). The ICD is currently in its 11th revision and functions under the name of ICD-11 for Mortality and Morbidity Statistics (ICD-11-MMS).  ICD-11 framework consists of three parts: Foundation (database), Classifications (attained from Foundation), and Biomedical Ontology (linked to Foundation). Biomedical ontology represents the most relevant knowledge of a disease that should be incorporated into the coding system. For example, when various manifestations of the COVID-19 disease were discovered after the release of ICD-11, they were able to be incorporated as new dimensions of the ICD-11 model.
International Classification of Functioning, Disability, and Health (ICF)[edit | edit source]
In 2001, the fifty-fourth World Assembly officially approved the International Classification of Functioning, Disability and Health (ICF) as a replacement for the International Classification of Impairments, Disabilities and Handicaps (ICIDH). The ICF is a classification of health and health-related domains. It focuses on human functioning, from “proper function” to “major disability”, in relation to a person’s activities and participation, which are influenced by environmental factors, health conditions, and personal factors.
The unique features of the ICF:
- Includes social and environmental aspects of disability and health
- Allows for the identification of factors at both individual and system levels
- Provides a framework for mental and physical disorders
- Allows for organisation and communication of information on human functioning
- Facilitates interdisciplinary and inter-professional practice by providing specificity and a common language in functioning and disability
- Allows for an interactive relationship between health conditions, impairments, functional limitation/activity restrictions and environmental and personal factors
- In this model, the relationship is not linear, but one component influences and is influenced by other factors 
- Data collection allows the user to determine the associations and causal links between different components
ICF and Rehabilitation[edit | edit source]
The role of the ICF in rehabilitation is evolving. The following are the current and potential implications of the ICF for rehabilitation:
- Concept of functioning and disability finds support in the assessment and documentation
- Improvement in understanding by rehabilitation professionals of all the domains of participation and how the environment influences participation
- There is also an improved understanding of when to include these domains when planning rehabilitation and anticipating outcomes
- There has been progress in enhancing universality and standardisation of functions, disabilities, and health
- A focus on removing environmental barriers when rehabilitation services are planned
- Researchers’ and professionals’ reasoning has evolved when considering statistics and information related to function, disability, and health
- Introduction of a holistic approach to the management of patients
Limitations of the ICF classification in rehabilitation:
- Limited applications by healthcare professionals, especially those outside of the rehabilitation team
- Limited training of an interprofessional team to adopt the ICF in daily practice
- Limited use of the ICF in the rehabilitation setting by vocational rehabilitation counsellors
Key Components of the ICF Model[edit | edit source]
The key components of the ICF model include health conditions or disorders/diseases, body function and structure, activities and participation, environmental factors, and personal factors. Due to cultural variability, personal factors are not included in the ICF classification. However, they are recognised as being essential for understanding functioning and disability.
The following definitions of the ICF components are provided in the manual:
Body Functions and Structures:
- Body Functions: “the physiological functions of body systems (including psychological functions)”
- Body Structures: “anatomical parts of the body such as organs, limbs and their components”
Impairments: “problems in body function or structure”. Includes a significant deviation or loss.
Activity: “execution of a task or action by an individual”
Participation: “involvement in a life situation”
Activity Limitations: “difficulties an individual may have in executing activities”
Environmental Factors: “physical, social and attitudinal environment in which people live and conduct their lives” Environmental factors can be considered a barrier or facilitation in terms of how they affect one’s functioning.
Personal Factors are not classified within the ICF but are recognised as a key to understanding functioning and disability. The following are examples of personal factors: age, gender, race, fitness, lifestyle, habits, upbringing, coping style, social background, education, character style, and important life events.
ICF Chapters[edit | edit source]
The first level of each of the core components of the ICF consists of chapters. Each chapter’s role is to provide an overview of the area of functioning. Next, chapters are classified into three levels that further describe the functioning area.
Table 1 shows the key components of the ICF and its chapters:
Table 1. The key components of the ICF Model and its chapters.
Clinical Application of the ICF Model in Acute Care[edit | edit source]
In acute care, physiotherapists and occupational therapists often work on the same task to facilitate a patient’s return to previous functions. These functions can be social, family or work-related. The ICF chapters on activities and participation can help clinicians explain to the patient, family, and insurance company the difference between the activities provided by the physiotherapist and occupational therapist.
Example: A patient is a young mother whose family role is caring for a 5-month-old child. She must be able to carry her child as she descends the stairs. A physiotherapist will work on improving the patient’s safety and endurance when walking on the stairs while carrying an object (mobility). The occupational therapist will have the patient complete the same task to prepare this patient for family demands (major life areas).
Final Introductory Thoughts on the ICF[edit | edit source]
The ICF “is the conceptual basis for the definition, measurement and policy formulations for health and disability. It is a universal classification of disability and health for use in health and health-related sectors.”
It is a valuable tool in research into disability in all the dimensions, including “impairments at the body and body part level, person level activity limitations, and societal level restrictions of participation” 
It is useful for healthcare professionals in helping to identify healthcare and rehabilitation needs and identifying and measuring the effect of the physical (built) and social environment on a person’s life. 
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