Disability Index (DI)

Abstract

The Disability Index (DI), introduced by Zhang et al. in 2023, is a 21-item psychometric inventory designed to provide a comprehensive measure of disability status and subsequent care needs among the older adults population. Conceptually, the DI was developed by integrating lessons learned from the Frailty Index (FI) and the Successful Aging Index (SAI). The scale assesses four critical domains of functioning: Activities of Daily Living (ADL), Instrumental Activities of Daily Living (IADL), physical performance, and cognitive function.

The development and initial validation of the DI utilized data collected from older adults aged 65 to 105 years across 22 provinces in China. The study demonstrated strong psychometric properties, reporting high internal consistency and support for convergent, divergent, known-groups, and concurrent validity. However, the initial report noted that test-retest reliability was not assessed and remains a recommended area for future psychometric evaluation.

Keywords

Activities of Daily Living, Cognitive Function, Disability, Divergent Validity, Instrumental Activities of Daily Living, Known-Group Validity, Older Adults, Physical Performance, Functional Status, Geriatric Assessment.

Authors

Zhang, Xuxi, Lin, Lizi, Sun, Xinying, Lei, Xiaoyan, Liu, Gordon G., Raat, Hein, Zeng, Yi.

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Purpose

The primary purpose of the Disability Index (DI) is to provide a standardized, robust tool for assessing the multidimensional nature of disability in the geriatric population. By consolidating measures of physical and cognitive functioning, the index aims to yield a single, quantifiable score that reflects an individual’s overall level of functional impairment.

Ultimately, the scale is intended for use in clinical and public health settings to identify the specific care needs among older adults, facilitating more accurate resource allocation, targeted support planning, and outcome monitoring in long-term care services.

Construct

The Disability Index measures the construct of **Disability Status** or **Functional Decline**, conceptualized as the cumulative deficit of impairments across essential daily living and cognitive tasks. The scale integrates 21 items across four distinct, yet related, domains, reflecting the complexity of aging and functional independence.

The four domains—Activities of Daily Living (ADL), Instrumental Activities of Daily Living (IADL), Physical Performance, and Cognitive Function—contribute to a total score that ranges from 0 to 100. This structure is designed to capture subtle variations in functional capacity, with higher scores indicating a more severe degree of disability.

Validity

The initial validation study confirmed multiple facets of the DI’s validity across the total sample and within specific age subgroups (65–79, 80–89, 90–99, and 100–105 years).

• Convergent and Divergent Validity: The convergent and divergent validity of the four subdomains were supported by their correlations with corresponding alternative measurements. This indicates that each domain accurately measures its unique construct while showing expected relationships (convergence) or lack thereof (divergence) with other related concepts.
• Known-Group Validity: Strong known-group validity was established for the full DI and its four subdomains. The scale demonstrated a clear discriminative ability, effectively distinguishing between groups of individuals known to have different levels of disability severity across all age cohorts.
• Concurrent Validity: The concurrent validity of the full DI was found to be excellent in the total sample, achieving Area Under the Curve (AUC) values of 0.8 or greater when predicting care needs. For individual age subgroups, concurrent validity was generally acceptable (most AUCs ≥0.7). However, when the criterion was defined as “Heavy informal care burden,” AUCs in the 65–79 years old and 100–105 years old groups were slightly lower, ranging from 0.64 to 0.69.

Reliability

Reliability of the Disability Index was primarily assessed through internal consistency, measured by Cronbach’s alpha. The results indicate high reliability for the overall index and its major subdomains.

• Total Sample Internal Consistency:
  • Full DI: 0.94
  • Activities of Daily Living (ADL) domain: 0.87
  • Instrumental Activities of Daily Living (IADL) domain: 0.95
  • Physical Performance domain: 0.72
• Subgroup Consistency: Within each age subgroup, the Cronbach’s alpha for the full DI, ADL, and IADL domains remained consistently strong (≥0.70). The Physical Performance domain showed acceptable, albeit lower, reliability, with Cronbach’s alpha values varying between 0.66 and 0.68 across all age subgroups. The developers recommend that future research should focus on assessing **test-retest reliability** to confirm stability over time.

Factor Analysis

No formal factor analysis, such as Exploratory Factor Analysis (EFA) or Confirmatory Factor Analysis (CFA), was detailed in the provided source material regarding the underlying dimensional structure of the Disability Index.

Instrument

Test Type: Original

Format: The DI is a 21-item questionnaire. Items related to ADL, IADL, and physical performance domains are typically rated on a 3-point scale, ranging from “able to perform the activity without difficulty” to “unable to perform the activity independently.” A specific exception is items 4–6 in the physical performance domain, which use a binary scale ranging from “both hands can perform the activity” to “neither hand can perform the activity.” Item scores for ADL, IADL, and physical performance are converted into index scores and averaged before being integrated into the full DI score.

The cognitive function domain utilizes the total score from the Chinese version of the Mini-Mental State Examination (MMSE). These MMSE scores are categorized and assigned fractional values to align with the DI’s 0–100 scale: severe impairment (score 0-17) is assigned 100; moderate impairment (18-21) is 67; mild impairment (22-25) is 33; and no cognitive impairment (26-30) is 0. The final total score for the full DI (21 items) ranges from 0 to 100, calculated by averaging the component scores, where higher values indicate greater levels of disability.

Language Available: English

Population Group: Human; Male; Female

Age Group: Adulthood (18 yrs & older); Aged (65 yrs & older); Very Old (85 yrs & older)

Population Details: The scale was validated using data collected from older adults, aged 65-105 years old, across 22 provinces in China.

Test Methodology: The methodology included comprehensive assessments of Test Validity (Concurrent Validity, Convergent Validity, Discriminant Validity) and Test Reliability (Internal Consistency).

Keywords

Activities of Daily Living (ADL), Instrumental Activities of Daily Living (IADL), Cognitive Ability, Functional Status, Geriatric Assessment, Physical Mobility, Internal Consistency, Concurrent Validity, Mini-Mental State Examination (MMSE).

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Authors

Author ORCID Identifier:

Zeng, Yi: http://orcid.org/0000-0001-8354-2945

Affiliation Email addresses:

Zeng, Yi: [email protected]

Correspondence Address:

Zeng, Yi: Duke University Medical School, Center for the Study of Aging and Human Development, 40 Duke Medicine Cir, Durham, North Carolina, United States, 27710, [email protected]

Permissions & Fee and Test Year

Permissions: Contact Corresponding Author

Commercial: No

Fee: No

Test Year: 2023

Reference’s

Zhang, X., Lin, L., Sun, X., Lei, X., Liu, G. G., Raat, H., & Zeng, Y. (2023). Development and validation of the Disability Index among older adults. The Journals of Gerontology: Series A: Biological Sciences and Medical Sciences, 78(1), 111–119. https://doi.org/10.1093/gerona/glac059

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Items of the Disability Index (DI)

The Disability Index is a 21-item measure composed of items categorized into four domains:

• Activities of Daily Living (ADL)

• Instrumental Activities of Daily Living (IADL)

• Physical Performance

• Cognitive Function

No data is Available regarding the specific content of the 21 items.