Intrinsic Capacity (IC)

Intrinsic Capacity (IC) (cross-sectional score)

LASA file name(s):
CWIC (1^st cohort), FWIC (2^nd cohort)

Contact: Natasja van Schoor

Background
The World Health Organization (WHO) developed the Integrated Care for Older People (ICOPE) framework as an innovative solution to the problems caused by the aging population [1]. One of the key concepts that form the basis of ICOPE is “intrinsic capacity” (IC), which is defined as the total of a person’s physical and mental abilities that contribute to healthy aging. IC is the composite of all the physical and mental capacities that an individual can draw on, covering five sub-domains: locomotion, cognition, vitality, psychology and sensory [2].

A cross-sectional IC score was created and validated based on data from the Longitudinal Aging Study Amsterdam (LASA) using a formative approach. Koivunen et al. [3] utilized multiple regression to identify the most relevant indicators linked to the IC construct, employing 6-year functional decline as an outcome. The score’s structural validity  was further assessed by evaluating whether the selected indicators represent all the five domains of the IC construct as well as known group and criterion validity of the constructed summary score.

Score development

Study sample
Data from the main and medical interviews of the first two LASA cohorts were combined, with baseline measurements in 1995/1996 (aged ≥ 65 years) and in 2005/2006 (aged ≥ 57 years), respectively. These measurement cycles were used for the baseline analyses, since not all relevant variables for operationalizing IC were available at the first LASA measurement cycles of the cohorts in 1992/1993 and 2002/2003, respectively. Follow-up outcome data on functional limitations were drawn from the measurement cycles conducted in 2001/2002 and 2011/2012, for the first and second cohort, respectively.

Candidate indicators
Measurements of indicators collected in LASA that fit best to the conceptualization of the IC construct were considered, to ensure the content validity of the score. Selection of potential indicators was guided by the following criteria:
the indicator
(1) has been identified as a predictor of health and functional decline during aging in prior literature,
(2) has preferably continuous scoring and is able to detect low and high capacities in one of the five defined key domains of IC,
(3) can be easily administered and incorporated in routine clinical practice, and
(4) is available at different LASA measurement waves, to have the opportunity to study changes in LASA-IC score over time in future research. The following indicators measured at baseline (1995/1996 for Cohort 1; 2005/2006 for Cohort 2) were considered to cover the five domains:

• Vitality was measured with hand grip strength;
• Locomotion was assessed with walking speed, chair rise test, and standing balance test (see physical performance);
• Cognition was assessed with memory, information processing speed, and general cognitive functioning;
• Psychology was measured with depressive symptoms, anxiety, mastery, and self-efficacy;
• Sensory was assessed with self-rated items of vision and hearing. Vision was assessed with 3 items: “Can you read the normal, small print in the newspaper without glasses or contact lenses?”, “Can you recognize someone’s face from a distance of 4 m without glasses or contact lenses?”, and “Can you see well enough?”. Hearing was assessed also with three items: “Can you follow a conversation in a group of three or four persons without an aid?”, “Can you follow a conversation with one person without an aid?”, and “Can you hear well enough?”.

Indicator selection
After stratification by sex, all the candidate indicators of IC were rescaled using the percent of maximum possible (“POMP”) method [4, 5], so that the variables had the same unit. After rescaling, all variables ranged from 0 (low capacity) to 100 (high capacity). , IC indicators were selected by prediction modelling and bootstrapping with 6-year functional decline as outcome [3]. A stepwise backward elimination procedure were used to exclude indicators that were not statistically significant (p > 0.05). To test stability of the indicator selection and combination of selected indicators, bootstrapping was performed with 2000 samples and further calculated the frequency of indicator and model selection.. The indicator were included in the IC score if it was selected in ≥ 50% of the samples.

Final selected indicators
After stepwise backward logistic regression, 7 of the 17 candidate indicators of IC were significantly associated with 6-year functional decline (Table 1). All the five domains of the IC construct were covered, and no domains had to be forced into the model [3]. Next, a mean score for domains with multiple indicators before calculating a mean score over all five domains were calculated. An individual’s IC score can range from 0-100, whereby higher scores indicate better IC.

Table 1. Selected IC indicators (by logistic regression model with functional decline as an outcome developed in combined data of 58–88-year-old people from the two LASA cohorts )

Previous use in LASA
Koivunen et al. [3] created this cross-sectional IC measure using data from LASA. By adopting a formative approach, an IC score was constructed (ranged from 0 to 100) based on seven indicators covering all five domains. The average IC score was 66.7, higher among younger participants and those with fewer chronic diseases. The IC score demonstrated discriminative ability based on age and health status and showed associations with functional decline and mortality. N.B. The number of respondents in [FILENAME] is different from the number of respondents  that is reported in the study by Koivunen et al. In that study, the final IC score was constructed based on complete cases in regards to all 17 pre-selected indicators,. In [FILENAME] the IC score was constructed based on complete cases in regards to the selected 7 indicators, which has led to  a bigger sample size compared to the study of Koivunen.

References

1. World Health, O., World report on ageing and health. 2015, Geneva: World Health Organization.
2. Cesari, M., et al., Evidence for the Domains Supporting the Construct of Intrinsic Capacity. The Journals of Gerontology: Series A, 2018. 73(12): p. 1653-1660.
3. Koivunen, K., et al., Development and validation of an intrinsic capacity composite score in the Longitudinal Aging Study Amsterdam: a formative approach. Aging Clin Exp Res, 2023. 35(4): p. 815-825.
4. Cohen, P., J. Cohen, L.S. Aiken, and S.G. West, The Problem of Units and the Circumstance for POMP. Multivariate Behavioral Research, 1999. 34(3): p. 315-346.
5. Moeller, J., A word on standardization in longitudinal studies: don’t. Frontiers in Psychology, 2015. 6.
6. Piccinin, A.M. and P.M.A. Rabbitt, Contribution of cognitive abilities to performance and improvement on a substitution coding task. Psychology and Aging, 1999. 14(4): p. 539-551.
7. Bosscher, R.J. and J.H. Smit, Confirmatory factor analysis of the General Self-Efficacy Scale. Behaviour Research and Therapy, 1998. 36(3): p. 339-343.

Date of last update: November 21,  2023 (first version)