The increasing survival rate from chronic diseases and decreasing birth rate are making Spain one of the fastest ageing societies in the world 12. The proportion of the population over 65 years of age doubled during the last 30 years and is expected to double again by 2050 3. The 1999 National Survey on Disability, Impairments and Health indicated that 59% of individuals with disabilities in Spain were aged over 65 years. Moreover, 33% of those over 65 were disabled as identified in this survey 4. Disability among the elderly people represents a major public health concern in Spain. National prevalence rates of disability and a quantitative approach to disability determinants are needed for service development and evidence-based health decision-making with regard to this population.

Over the last few decades, researchers have used a variety of approaches to disability measurement. These varied strategies have hampered comparability across studies and have rendered highly variable epidemiological conclusions on the distribution of disability 5. Previous enquiries have frequently focused on dependence for activities of daily living (ADL, e.g., eating, moving around the home; cf., Katz index, Barthel index), and sensory and cognitive functions 46. By contrast, the International Classification of Functioning, Disability and Health (ICF) 7 incorporates a multifactorial approach to disability with two core components: limitations in activities and participation, and changes in body structure and functions. Under the ICF model diseases, environmental factors, and personal characteristics can all function as determinants of disability. According to this framework, an individual's functional status results from the interaction between the health problems and the physical, social, and psychological context of the individual.

The ICF provides the basis to develop disease-specific disability profiles 8. Moreover, the ICF facilitates the identification of targets in rehabilitation, assessment of intervention outcomes, and social and health service planning 9. For instance, ICF-based models of assessment, as the World Health Organization Disability Assessment Schedule 2^nd Edition for epidemiological use (WHO DAS-II) 10, pinpoint specific spheres of the individuals personal and social functioning, which may be linked to specific needs of social support. Along these lines the ICF checklist for clinical use 11 provides a multi-faceted classification of the components of disability, the impact of environmental aids and the composition of the package of services that may best suit the needs of a particular individual.

The ICF scheme opens up the possibility of cross-national and multi-dimensional assessment of disability, thereby offering a more extensive picture of which aspect may be affected for any given individual or cohort. The WHO-DAS II has been developed under the ICF scheme and may constitute an effective tool for epidemiological surveys. WHO-DAS II is well suited for prevalence studies implementing door-to-door designs given that both the screening (12 items) and the full (36 items) versions of this instrument are available 1213. Traditional approaches to disability assessment incorporate items relevant to disability, particularly for clinical use in nursing (e.g., Katz index, Barthel index). However, they frequently amalgam items on function (e.g., digestive and urinary function, sphincter control, motor function) without any indication of impact on social functioning. By contrast, WHO-DAS II provides a systematic approach to various social and functional domains of disability, which are assessed separately over a single 5-point Likert scale. As an additional advantage, WHO-DAS II has been designed specifically for epidemiological use being available in various languages and formats 14.

There have been no previous population-based studies on health and functioning in the Spanish elderly population under the ICF framework. The ICF system may provide a more comprehensive framework to disability witch could potentially become a standard for psychological assessment 15.

The goal of the present study was to obtain prevalence estimates of disability levels in a Spanish elderly population using WHO-DAS II (12 items) as a screening tool, WHO-DAS II (36 items) as the basis for a multi-faceted assessment of disability, and ICF disability severity ranges to define cases of mild, moderate, and severe/extreme disability.

The final sample comprised 546 participants of which 440 were positive to the disability screening and 106 scored 0 in WHO DAS II (12 items). Of this total, 503 had complete datasets (mean age: 82.0, SD: 4.8; 62.6% women). Most participants were of rural-mixed, urban, and sub-urban origin, mid or mid-low social class, and had a low level of education. Two-thirds of the sample was living in rural areas (municipality size ≤ 10000 inhabitants). The actual proportion in the whole country is slightly inferior (≈60%; source: Spanish National Institute of Statistics, http://www.ine.es/censo_accesible/es/seleccion_colectivo.jsp?fType=1), which may have bias slightly our results in terms of disability. The socio-demographic characteristics of the study population are shown in Table 2. Prevalent health conditions and morbidity of study sample is described in detail on Table S1 (online additional file 1).

Figure 2 shows a flow chart portraying the sample attrition. A total of 503 computable datasets were available. Non-computable scores were due to incomplete information and lack of collaboration during the second visit (n = 43). A total of 49 participants had dementia; among them only one score 0 on the disability screening. Seven out of 48 dementia cases scoring within the positive range of the disability screening did not have computable scores at the WHO-DAS II (36 items).

This study is among the few population-based surveys reporting disability prevalence assessed according to the ICF framework 2829. Our results provide a novel view of disability among the Spanish elderly population. Based on WHO-DAS II assessment and ICF disability categories, our study estimated the prevalence of mild, moderate, and severe/extreme disability for those aged 75 years and above. Prevalence rates of severe/extreme disability equalled 10.14%, which works out to a number of 390,000 individuals older than 75 in Spain meeting the case definition used in this study. While most participants displayed some degree of disability (2 out of every 3), the proportion of severely disabled individuals was approximately one tenth of the study sample. These figures are in line with prior studies conducted on European populations using various approaches to the assessment of disability 630. Given the mixed-rural and urban sample in the present study, we should comment on the impact of habitat on dementia. In a recent re-analysis of studies on the prevalence of dementia in Spain, suburban populations showed higher prevalence of Alzheimer's disease while vascular dementia was higher among suburban populations. The pattern was not replicated among urban-mixed populations and there is a paucity of evidence for rural populations 18.

The age-standardised prevalence of disability found in our study was considerably higher than the one reported by the 1999 National Disability Survey for the same age groups 3 (64% vs. 46% of persons aged ≥ 75 with some disability). The fact that our study population was consisted of samples from nine different locations may in part be a protection against selection bias, although our study sample cannot be considered representative of the country's population. Moreover, it should be noted that participants were recruited from the Central-Northern half of the country, which registers lower mortality from all causes 31 and higher longevity and quality of life than does Southern Spain 32. It should be noted, however, that the presence of dementia cases among participants with non-computable disability data might have reduced our prevalence figures (cf., Alzheimer's type dementia is one of the major disability determinants in the elderly 33). In addition, cognitive function of participants with non-computable WHO-DAS II was probably slightly inferior than for those with computable scores. Only 25 among the 41 participants with non-computable WHO-DAS II had computable MEC scores, and among those, cognitive function was significantly lower (23.58 ± 10.01 vs. 27.66 ± 6.18; Wilcoxon W, p = .033). These differences are of moderate-low magnitude and within standard criteria for computable scores 34.

The highest functioning level was found for GAP with little variation due to age, suggesting that basic social skills are preserved until very late in life. PSO on the other hand, which evaluates more advanced social skills, showed greater deterioration with age. A similar pattern was observed for SCA and LAC, as these are related to basic and instrumental activities of daily living, respectively. Men and women differed in GAR and LAC domains. Activities in both areas require high cognitive and motor functioning suggesting that the higher prevalence of age-related musculoskeletal and mental disorders (particularly arthritis, osteoporosis and dementia; see Table S1; online additional file 1) may underlie the difference in GAR and LAC performance between women and men.

Multi-dimensional assessment of disability (activities and participation) based on WHO-DAS II opens up a more comprehensive view of disability. The performance of WHO-DAS II as an instrument to inform activities and participation was satisfactory. However, specific aspects of the scale need to be developed further. For instance, the low acceptance of the sexuality item, suggest that this content (among others that are not applicable to all elderly population) needs to be reworded. In addition, WHO-DAS II does not assess the causes of disability. Therefore it is not highly useful to develop strategies to remediate disability. In this respect, ICF core sets represent a recent expansion of ICF framework for the assessment of disease-specific factors of disability. Core sets are composed of clusters of items that are distinctively affected under specific health conditions.

Previous studies using the WHO-DAS II have focused on younger and more specific clinical populations, particularly psychiatric patients 10353637. Our study illustrates the usability of the scale in population-based studies with elderly participants. The WHO-DAS II allows assessing concurrently various aspects of disability, and could be used as the basis for identifying complex relations between disability, and medical, personal, social and environmental factors 38.

The development of methodological strategies for the assessment of disability is highly relevant to clinical epidemiology. Until recently, instruments measuring disability have emphasized clinical profile over behavioral, social and environmental factors 39. More specifically, the validity of classic measures (e.g., Katz Index, Barthel Index) could be tested against ICF-based approaches to assessment (e.g., ICF checklist) in order to determine the relative contribution of personal, environmental and social factors on individuals' performance.

A study of this nature requires ICF-based instruments that are yet to be developed that would incorporate a relevant core set of items relevant to the characteristics of the disability of the population studied. For instance, this approach has been found effective in assessing the rehabilitation of patients with stroke 40. The level of morbidity found in elderly populations (Table S1; online additional file 1) suggest the necessity of a generic core set for elderly people allowing an in-depth assessment that would facilitate tailoring services for a particular individual. Our findings are descriptive in nature. Therefore, no social policy guidelines can be drawn from them. Nonetheless, our results are consistent with setting the prevention and remediation of severe/extreme disability in mobility and life activities as a priority for service planning. Strategic actions would require a more thorough examination of disability causes, which are currently the focus of our team.

Although a number of studies assessing disability in various settings have used the ICF system and the WHO-DAS II, they have been rarely used as the basis for screening and assessment in a population-based study on the prevalence of disability. This approach may be superior to more classical means of determining functional status (e.g., ADL, motor and sensory functioning), which do not allow a multi-faceted appraisal of disability. Our study shows that individuals perform differently in the various areas of disability assessed. Advanced social skills and instrumental activities of daily living, as assessed through the PSO and LAC domains of WHO-DAS II, demonstrated a higher deterioration with age as opposed to other areas of disability. Women tended to perform poorer in areas requiring high motor functioning, as is the case for the GAR and LAC disability domains. The level of specificity achieved by using ICF disability levels and WHO-DAS II may provide essential information for a more comprehensive development of services matching the needs of elderly people.

ADL: Activities of daily living; CI: Confidence interval; GAP: Getting along with people; GAR: Getting around; ICF: International Classification of Functioning a Disability; LAC: Life activities; MMSE: Manitoba Medical Services Foundation; OR: Odd ratio; PSO: Participation in society; SCA: Self-care; UAC: Understanding and communication; WHO-DAS II: World Health Organization-Disability Assessment Schedule 2^nd edition.

This project was partially funded by a research contract in support of the project "Epidemiological Study of Dementia in Spain" signed by the Pfizer Foundation and Carlos III Institute of Health. Funding agencies did not affect the design, implementation, or reporting of this study in any way.

JVO: Conducted the analyses, and wrote the first version of the draft. JPC: Designed the study and wrote various sections of the manuscript. MSM, PS, FSS, FRP, FR, RM, PMM, IM, JGB, JGO, FJG, GFM, FBP, AB, JAI, and JLB: Contributed equally to the study design, data collection, and manuscript writing. All authors read and approved the final manuscript.