In the ICF, there are different components of human functioning (and disability): body functions and body structures classify functions and structures at the organ system level respectively. An example of body function would be “muscle power function” and “structure of the shoulder region” would be body structure. Activities and participation classify the full range of actions, tasks, and social or life roles such as reading, carrying out daily routine, walking, and remunerative employment. Body function, body structure, and activities and participation can be influenced by characteristics of the person (personal factors) such as coping and his or her physical, social, and attitudinal environment (environmental factors) such as physical accessibility of a building, attitude of family members, and support from health professionals. Each ICF component, except for personal factors, is assigned a letter code: “b” for body functions, “s” for body structures, “d” for activities and participation, and “e” for environmental factors. Personal factors while defined as the “… background of an individual’s life and living, and comprise features of the individual that are not part of a health condition or health states” [2] are not classified (which means no codes) at this time.

Each ICF component consists of different chapters or domains (e.g. in body functions the chapters include mental functioning, sensory functions, functions of the cardiovascular, haematological, immunological, and respiratory system, etc.), and each chapter is made up of several alphanumerically coded ICF categories which are the specific units of a domain. Each ICF category is given a distinct alphanumeric code that identifies the component (b, s, d, or e), chapter (number), and level (specific domains) in the hierarchical structure. The classification and coding structure is presented in Fig. 2.3.

As previously said, each ICF category is assigned a component letter and numerical code which makes each category unique. The hierarchical arrangement is illustrated below under body functions within the domain “pain”:

Below is an example of the hierarchy of codes under body structure within the domain “structure of lower extremity”:

In some cases, 4th level categories are not available for some domains. Here is an example for activities and participation:

Below is an example of the hierarchy of codes under environmental factors within the domain “products and technology”:

As illustrated above, 3rd and 4th level categories are specifications of the more general and higher levels, namely, the 2nd and 1st levels. In the entire ICF, there are 30 chapters in total and 1,424 separate categories distributed across the four ICF components (body function, body structure, activities and participation, and environmental factors).

Table 2.1 illustrates the depth and breadth of coverage of the ICF at the chapter level. There are eight chapters for body functions ranging from mental functions to integumentary functions and also eight chapters for body structures (covering all body organ systems). Activities and participation has nine chapters ranging from the simple, person level (learning and applying knowledge) to the more complex, societal level (community, social, and civic life) of interaction. Finally, environmental factors cover the entire physical, human-built, technological, attitudinal, and social and political world which are divided into five chapters (Table 2.1).

Table 2.2 illustrates the specification of a chapter, e.g. on mental functions under the body functions component, where the two parts of the chapter relate to global mental functions, e.g. consciousness and intellectual function, and to specific mental functions, e.g. perceptual and higher-level cognitive functions.

Table 2.3 illustrates the specification of a chapter, Chap. 4 Mobility of the Activities and Participation component in this case. From this table, mobility is categorized into several mobility-relevant descriptions such as those of body position, handling objects, walking, and using transportation. Each category is defined in the ICF handbook, and inclusion and exclusion criteria for each are also provided to make the distinction between and among seemingly similar ICF categories. Readers are referred to the ICF handbook for the detailed description of ICF categories [2].

The previous section of this chapter provided what domains of functioning and disability are to be assessed in light of health condition. In this section, a way of assessing the problem in a given ICF category will be discussed using “ICF qualifiers” (rating scale). The ICF qualifiers can be used to rate the severity or magnitude of the impairment of body functions or body structures, limitation in activity, and restrictions in participation. The ICF also provides qualifiers to indicate the extent of an environmental factor being a barrier or a facilitator of functioning. Without these qualifiers, an ICF code would not be meaningful in terms of the information that it conveys.

There are different levels of the ICF qualifier. In this chapter, we will simplify and focus on the first-level qualifier and, as for the other levels, see the ICF handbook for details. The first-level qualifier is a generic rating scale from 0 to 4, with 0 = no problem, 1 = mild problem, 2 = moderate problem, 3 = severe problem, and 4 = complete problem. Two additional options can be used: 8 (not specified)² and 9 (not applicable).³ On the other hand, the ICF qualifier for the environmental factors has nine response options ranging from 4 (complete barrier) to +4 (complete facilitator), with a zero value indicating neither a facilitator nor a barrier. Three additional options for environmental factors can be used: 8 (barrier, not specified), +8 (facilitator, not specified), and 9 (not applicable) using the same principles of definition stated earlier [2]. For each ICF qualifier, the WHO also provides a corresponding range of percentage. See Table 2.4 for a summary of the ICF qualifiers.

So, for example, an ICF code with qualifier of “b134.1” means that there is a mild (or between 5 % and 24 % level of) impairment of sleep functions. The “b134” refers to the ICF code on sleep functions and the “1” refers to the ICF qualifier for mild impairment. ICF categories belonging to activities and participation require a performance and capacity qualifier which means at least two first qualifiers. An example is “d4300.32” which means that there is a severe (50–95 %) difficulty with performance in lifting (d4300) and moderate (25–49 %) difficulty with capacity in lifting. The first of the two qualifiers refers to performance and the second to capacity. The concept of performance refers to what an individual does in his or her current environment or actual context in which they live; performance involves the influence of environmental factors. Capacity qualifier refers to an individual’s ability tested in a standard or uniform environment (i.e. adjusted for environment) [2].

For environmental factors, a plus sign is used to denote that that environmental factor is a facilitator and no sign to denote a barrier. So, for example, a code of e330. + 4 means that support and relationship with “people in positions of authority” (i.e. e330) is a complete (96–100 %) facilitator, while a code of e330.4 means that the support is a complete barrier.

Readers are advised to consult the ICF handbook for more details on the ICF qualifiers.