Intrinsic Capacity (IC)

Intrinsic Capacity (IC)

LASA file name(s):
Cross-sectional composite IC score:
LASACWIC (1st cohort), LASAFWIC (2nd cohort)
Longitudinal IC scores (including domain-specific scores and  composite scores):
LASAZWIC1 (1^st cohort, waves C, D, E, F), LASAZWIC2 (2^nd cohort, waves F, G, H, I)

Contact: Natasja van Schoor

1. Background

The World Health Organization (WHO) defines healthy aging as maintaining the functional ability that supports well-being in older age, which is closely tied to an individual’s intrinsic capacity (IC). 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 [1]. Unlike disease-focused approaches, IC emphasizes function and has been recognized as a key indicator of an older person’s ability to cope with life’s demands. Both cross-sectional composite IC score and longitudinal IC scores (including domain-specific and composite IC scores) were created in LASA.

For the cross-sectional composite IC score, Koivunen et.al [2] used 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 groups and criterion validity of the constructed summary score.

For the longitudinal IC scores, separate factor analyse and Partial Least Squares Structural Equation Modelling were used to select indicators to construct the scores [3]. In a recent scoping review by our group, it has been suggested that IC should be examined as a formative construct [4]. Specifically, the formative approach means that individuals’ total IC is constituted by their capacities regarding vitality, sensory, cognition, psychology and locomotion [1, 2, 4]. As a result, each domain contributes to the overall construct of IC, and changes in any domain can affect the total IC. Given this formative nature of IC, which encompasses five distinct domains, selecting indicators for longitudinal IC measures were done at the domain-specific level to indicate the contributions of each domain [5]. An advantage of this method as compared with the method of Koivunen et al. is that the distinct dimensions of the IC construct can be defined on beforehand. The construct validity (score across age groups) and criterion validity (relationship with change in functional limitations) of these scores were assessed.

 

2. Score development

2.1 Cross-sectional composite IC score

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 [6, 7], 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. 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. All the five domains of the IC construct were covered, and no domains had to be forced into the model [2]. 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 )

IC domain	Indicator	Measurement in LASA
Vitality	Grip strength	Grip strength is measured each wave during the medical interview using a grip strength dynamometer (Takei TKK 5001, Takei Scientific Instruments Co. Ltd., Tokyo, Japan).
Locomotion	Walking speed	Walking speed was measured as time (seconds) needed to walk 3 m, turn around, and then walk back 3 m as fast as possible.
<idem>	Balance	Balance was measured with feet in the tandem position for a maximum of 10 s.
Cognition	Coding	Coding was measured by adjusted version of the Alphabet Coding Task, which is a letter substitution task [8].
Sensory	Vision: distance	This item was measured by asking the participants whether they can recognize someone’s face from a distance of 4 m without glasses or contact lenses.
<idem>	Hearing: following conversation in a group	This item was measured by asking the participants whether they can follow a conversation in a group of three or four persons without an aid.
Psychology	Self-efficacy	Self-efficacy was measured with a 12-item version of the General Self-Efficacy Scale (GSES-12)[9] .

 

Previous use in LASA
Koivunen et.al [2] 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. In the CWIC (1st cohort), FWIC (2nd cohort) 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.

2.2 Longitudinal IC scores

Study sample
Data from the first and second cohort were combined. Their measurements from 1995/96 and 2005/06 were used as the baseline measures. For the first cohort, follow-up measurements were performed in 1998/99, 2001/02, and 2005/06. For the second cohort, follow-up measurements were performed 2008/09, 2011/12, and 2015/16, respectively. With the combined cohorts, two study samples were used: an indicator selection sample and a score construction sample. The indicator selection sample included individuals who participated in both the general and medical interviews at baseline as some potential indicators were collected in the general interview and others in the medical interview. For the score construction sample, a larger sample of 3246 participants at baseline was used, aged 55 and over, including 2,372 from the first cohort and 874 from the second cohort. This sample is larger in comparison to the indicator selection sample, as some missings on the indicators were allowed when constructing the domain-specific scores (see section IC scores construction), but not during the selection of indicators for the domains.

Steps of IC scores development and validation

Step 1. Indicators were selected from five IC domains: vitality, sensory, cognition, psychology, and locomotion. Candidate indicators were identified through literature review, past work, and expert opinions, and their presence in LASA dataset. Compared to the indicators selected by Koivunen et al., we considered new candidate indicators in light of recent developments from the WHO and new literature. The inclusion of indicators under the vitality domain was guided by the working definition of vitality. Hand grip strength [2, 10-13], peak flow [10, 14, 15], calf circumference [16], appetite [17], sleep quality, and self-reported weight change [17] were considered to construct the vitality domain. These indicators cover several important attributes of vitality. Hand grip strength and peak flow measure neuromuscular function. Calf circumference measures body composition [18]. Appetite was chosen as age-related physiological changes, such as decreased ghrelin release in the stomach, are known to increase feelings of fullness and reduce appetite [19]. Self-reported weight loss assesses the nutritional aspect and sleep quality evaluates energy levels. Self-rated items on hearing in a conversation, being able to use a normal telephone, near vision, and far vision were considered to construct the sensory domain [13]. General cognitive functioning [20], information processing speed [2], and episodic memory [2] were considered to construct the cognition domain. These performance-based measures are commonly used as key indicators within the cognition domain [10]. Anxiety symptoms [20] and depressive symptoms [20] have been commonly used as indicators for the psychology domain. It is also important to question whether the absence of anxiety or depressive symptoms fully captures the spectrum of psychological capacity, particularly on the positive end. Research suggests that resources related to a sense of control and the ability to mentally adapt to adversities can be preserved or even enhanced through growth, experiences, and learning during ageing [21, 22]. Therefore, we have also included mastery [23], self-efficacy [2], and self-esteem [24]as these capacities may be essential in compensating for physiological losses. Walking speed [10, 25], chair rise test [12], standing balance [10], and cardigan test were considered to construct the locomotion domain. These are commonly used performance-based measures for assessing locomotion [10].

Step 2, in the indicator selection sample, unidimensional factor analyses and Partial Least Squares Structural Equation Modelling (PLS-SEM) were used to select indicators to construct domain-specific IC scores. All  candidate indicators of IC were first rescaled using the percent of maximum possible (“POMP”) method [6, 26]. After rescaling, all indicators ranged from 0 (low capacity) to 100 (high capacity). Next, separate unidimensional factor analyses were performed on each of the five domains of IC. Indicators with loading greater than 0.40 were selected for further analysis [27]. Selected indicators were then incorporated into a PLS-SEM model. The structural model estimated the relationships between latent constructs (i.e. the five IC domains), with correlations between the five domains being estimated. For this, indicator correlation weights represent each indicator’s relative importance to the construct and indicator loading represents the absolute contribution of an indicator to its construct [28] [29]. It is also recommended to consider the absolute contribution of a formative indicator to the construct, which is determined by the formative indicator’s loading [30]. In general, indicator loadings of 0.50 [30] and higher suggest the indicator makes a sufficient absolute contribution to forming the construct, even if it lacks a significant relative contribution [29]. Significance of the indicator weights was based on 10,000 bootstrap samples [31].A total of 18 indicators were selected:

 

Table 2.

IC domain	Indicator	Measurement in LASA
Vitality	Grip strength	Grip strength is measured each wave during the medical interview using a grip strength dynamometer (Takei TKK 5001, Takei Scientific Instruments Co. Ltd., Tokyo, Japan).
<idem>	Peak flow	Peak expiratory flow rate (PEFR, also known as peak flow) is defined as a person’s maximum speed of expiration [14].
<idem>	Calf circumference	Calf circumference was a simple anthropometric measure that highly correlates with muscle mass.
Locomotion	Walking speed	Walking speed was measured as time (seconds) needed to walk 3 m, turn around, and then walk back 3 m as fast as possible.
<idem>	Chair rise test	In the chair rise test, participants folded their arms across the chest, and the time to perform five sit-to-stand rises was measured in seconds.
<idem>	Cardigan test	For testing the ability to put on and take off a cardigan, the time required to put on and take off a cardigan, which was brought in by the interviewer, was scored.
Cognition	Information processing speed	Coding was measured by adjusted version of the Alphabet Coding Task, which is a letter substitution task [8]..
<idem>	General cognitive functioning	General cognitive functioning was measured with the Mini-Mental State Examination (MMSE) [32].
<idem>	Episodic memory	Episodic memory was measured with a 15 Words Test (15WT), which was a Dutch version of the Auditory Verbal Learning Test [33, 34].
Sensory	Far vision	This item was measured by asking the participants whether they can recognize someone’s face from a distance of 4 m without glasses or contact lenses.
<idem>	Near vision	This item was measured by asking the participants whether they can read the normal, small print in the newspaper.
<idem>	Use normal telephone	This item was measured by asking the participants whether they can use a normal telephone.
<idem>	Hearing in a conversation	This item was measured by asking the participants whether they can follow a conversation in a group of three or four persons without an aid.
Psychology	Anxiety symptoms	Anxiety was measured with the anxiety subscale of the Hospital Anxiety Depression Scale (HADS-A) [35]
<idem>	Depressive symptoms	Depressive symptoms were assessed with the Center for Epidemiologic Studies Depression Scale (CES-D) scale [36].
<idem>	Mastery	Mastery was measured with the five-item version of the seven-item Pearlin Mastery Scale [37, 38].
<idem>	self-efficacy	Self-efficacy was measured with a 12-item version of the General Self-Efficacy Scale (GSES-12)[9]
<idem>	self esteem	Self-esteem was measured by an adapted version of the Rosenberg Self-esteem scale [39]

Step 3, At each measurement wave , a mean score (domain-specific score) was first calculated for each domain using the 18 selected indicators. We applied the general rule that the mean domain-specific score was calculated when 50% or more of the indicators were present [40]. Subsequently, the composite IC score was computed by averaging the five domain-specific scores, but only if all five domain-specific scores were present. All computed scores are reported in POMP units, ranging from 0 to 100.