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RESEARCH ARTICLE Open Access

Disability incidence and functional decline
among older adults with major chronic
diseases
Joelle H. Fong

: More than 80% of elderly Americans have at least one chronic disease. While past studies have
shown that hierarchical patterns of functional loss may differ by gender and institutional settings, little is known
about whether such patterns differ in relation to chronic health condition. The aim of this study is to investigate
the pattern of functional loss among older adults with major chronic illnesses, and to compare their onset and
ordering of incident ADL disability with those of persons without such conditions.

: We use a nationally representative sample of persons aged 80+ from the 1998–2014 Asset and Health
Dynamics of the Oldest Old survey. The group with major noncommunicable diseases (including cardiovascular
disease, cancer, chronic respiratory disease, and diabetes) comprises 3,514,052 subjects, while the comparison group
comprises 1,073,263 subjects. Self-reports of having difficulty with six distinct ADLs are used to estimate disability
incidence rate. Nonparametric statistical methods are used to derive median onset ages and ADL loss sequence
separately for each group.

: Older adults with major chronic diseases have higher rates of incident disability across all ADL items.
Estimated median onset ages of ADL disabilities for the full sample range from 91.5 to 95.6. Disability occurs earlier for
chronically ill persons (onset ages 91.1–95.0) than for those in the comparison group (onset ages 93.5–98.1). Among
those with major chronic diseases, the ADL loss sequence ordered by median ages of disability onset is bathing,
walking, dressing, toileting, transferring and eating. The activities are also distinctly separated into an early-loss cluster
and a late-loss cluster. Although the loss sequence derived for the comparison group is largely similar, disability
progression for those with major chronic diseases is compressed within a shorter timeframe and the timing gaps
between adjacent disabilities are smaller.

: Older Americans with major noncommunicable diseases face an earlier and steeper slope of functional
decline. Chronic care delivery programs should adapt to dynamic changes in older patients’ functional status. Health
interventions to help patients delay disability onset and optimize functional autonomy within emerging models of
chronic care should especially target early-loss activities such as bathing, dressing, and walking.

Keywords: Aging, Disability incidence, ADL disability, Oldest old, Longitudinal research

JEL classification: G22, H51, H75, C24

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Correspondence: j.fong@nus.edu.sg
National University of Singapore, 469C Bukit Timah Road, Singapore 259771,
Singapore

Fong BMC Geriatrics (2019) 19:323
https://doi.org/10.1186/s12877-019-1348-z

http://crossmark.crossref.org/dialog/?doi=10.1186/s12877-019-1348-z&domain=pdf

http://orcid.org/0000-0003-0267-6779

http://creativecommons.org/licenses/by/4.0/

http://creativecommons.org/publicdomain/zero/1.0/

mailto:j.fong@nus.edu.sg

Background
The onset of functional disability is a dynamic and pro-
gressive process. As one ages, health problems accumu-
late and people start to lose their ability to perform
activities of daily living (ADLs), such as dressing, using
the toilet, bathing, and eating. Past studies have shown
that the pattern of ADL disability in geriatric popula-
tions follows a distinct progression [1–9]. These hier-
archical patterns of functional decline or ADL scales –
typically established using item response theory (IRT)
methods and hazard models – have been reliably and
validly assessed not only for the institutionalized popula-
tion but also more generally for community-dwelling
older adults [7]. Several studies have also highlighted
that disability progression differ by gender [3]; time pe-
riods [6]; institutional settings (e.g. residential setting,
nursing home) [6]; types of ADL items [4, 10]; and coun-
tries [11, 12]. Yet, little is known whether the pattern of
functional loss differs in relation to chronic health
condition.
A closer examination of the pattern of functional loss

among older persons with major chronic illnesses is
valuable for two reasons. First, there is strong theoretical
basis that functional disability onset is driven by physio-
logical changes associated with aging and underlying
chronic diseases [13]. This notion has been borne out
empirically in prior studies which demonstrate strong
associations between the increased incidence of func-
tional disability and chronic diseases such as diabetes,
stroke and heart disease among the elderly [13–15]. Ac-
cordingly, the onset, ordering and general pattern of in-
cident ADL disability among chronically ill persons may
be distinct from their healthier peers. Second, chronic
diseases are among the most prevalent and costly health
conditions in the United States. 85% of Americans over
65 years of age have at least one chronic health condi-
tion and 60% have at least two chronic conditions [16].
In particular, cardiovascular diseases (CVD), cancers,
chronic respiratory diseases and diabetes impose a dis-
proportionate impact on the overall disease burden.
Known as the ‘big four’ noncommunicable diseases
(NCDs), these four conditions are the leading causes of
disability and death in the United States [17–19].
The purpose of this study is to investigate the pattern

of functional loss among older adults with major chronic
illnesses, and to compare their onset, ordering and gen-
eral pattern of incident ADL disability with those of per-
sons without such conditions. Major chronic diseases
are defined to include CVD (stroke, heart attack, and
heart diseases), cancer, diabetes and chronic lung dis-
ease. We use a nationally representative sample of
oldest-old adults aged 80 and older from the 1998–2014
Asset and Health Dynamics of the Oldest Old (AHEAD)
study. Respondents interviewed at the 1998 baseline are

followed across 10 survey waves. We divide respondents
into two groups (those who had or developed a major
chronic disease during the observation period versus
those who did not). For each group, we first report the
cumulative incidence rates of disability and then derive
age distributions of disability onset by ADL item using
nonparametric statistical methods. Median ages of inci-
dent disability drawn from the distributions are used to
identify the hierarchical ADL loss sequence for those
with and without major chronic diseases.
Closest to this present study, Dunlop et al. [3] demon-

strate that multiple waves of survey data can be pooled
together to evaluate the hierarchy of disability. In that
study, the authors used data from the 1984–1990 Longi-
tudinal Study of Aging and found that the ADL loss se-
quence ordered by median ages of disability onset was:
walking, bathing, transferring, dressing, toileting, and
eating. Some studies also explore using IRT methods on
longitudinal data. One study which applied the Rasch
scaling model document an ADL hierarchy of bathing,
dressing, transferring, toileting, walking, and eating
among adults aged 85 and above from the 1983–96
Aging in Manitoba Longitudinal Study [6]. Also using
the Rasch model, Fong and Feng [12] report a somewhat
similar sequence (bathing, walking, dressing, toileting,
transferring, and eating) based on data from the 1998–
2008 Health and Retirement Study. Accordingly, it is
useful to assess whether the functional loss sequences
derived in this present study are consistent with these
documented patterns of functional decline for older
Americans.

Methods
Data and measures
Data is obtained from the 1998–2014 Asset and Health
Dynamics of the Oldest Old study. The AHEAD is a
prospective panel study of older Americans born in 1923
or earlier, and respondents are interviewed every 2 years
since 1992. We set 1998 as the baseline because ADL
question wordings and response coding for AHEAD re-
spondents was made consistent only when the AHEAD
merged with the Health and Retirement Study that year
[20]. The AHEAD contains detailed information on
sociodemographic characteristics, family structure, phys-
ical health, cognition, and living arrangements. The 1998
AHEAD survey covered 5951 respondents aged 75 and
above; a complete description of the AHEAD is given
elsewhere [21, 22]. Our study sample comprises 1604
older adults aged 80+ who have zero ADL disabilities in
1998, and complete health and mortality data for all
follow-up waves. This represents about 27% of the 1998
AHEAD cohort. The weighted study population totals 4,
587,315.

Fong BMC Geriatrics (2019) 19:323 Page 2 of 9

Functional disability is measured by self-reports of
having difficulty with basic self-care tasks. AHEAD re-
spondents are asked: “Because of a health or memory
problem do you have any difficulty with [ADL]?, where
[ADL] refers to dressing; walking across a room; bathing;
eating; getting in and out of bed; and using the toilet.”
The responses to each ADL item are coded as six di-
chotomous variables in each wave [23]. The AHEAD
survey also collects information on a set of doctor-
diagnosed health problems and chronic conditions, in-
cluding the ‘big four’ NCDs, in all waves. Respondents
are asked, “Has a doctor ever told you that you have had
a [chronic condition]?” Those who responded affirma-
tively to questions relating to heart disease, stroke, can-
cer, diabetes, and chronic lung disease in 1998 or at any
point during the observation period are thus categorized
as persons with major chronic illnesses. Based on this
classification, the number of respondents with major
NCDs is 1203, while the number of respondents without
these NCDs is 401.

Statistical analyses
To determine whether individuals with major chronic
conditions are at higher risk of disability onset than their
peers, we calculate the proportion of new cases of dis-
ability (e.g. bathing) and report the total disability inci-
dence rates by ADLs for each risk group. The weighted
population of the group with major chronic illness com-
prises 3,514,052 subjects (unweighted: 1203), while the
comparison group comprises 1,073,263 subjects (un-
weighted: 401). Death rates by risk group and chronic
health status are also evaluated. Discrete-time hazards
models are then used to evaluate the age distributions of
disability onset and ADL ordering for each group. Spe-
cifically, following Dunlop, Hughes, & Manheim [3], we
utilize the nonparametric Turnbull [24] algorithm which
relies on an iterative procedure to estimate the failure
probabilities at discrete time points.
The individual is the unit of analysis. Binary variables

are created for each ADL (e.g. bathing) to indicate
whether or not a subject developed that disability during
the follow-up period. The Turnbull procedure is suitable
since the disability data on hand for each subject ob-
served is interval-censored. That is, disability was moni-
tored at 2-year intervals so exact time of disability onset
is unknown. Thus, for instance, an 85-year old respond-
ent who has no difficulty bathing in 2000, fails to re-
spond in 2002, then reports needing help with bathing
in 2004 will be assigned a bathing disability onset inter-
val of age 85–89. Respondents who do not have that dis-
ability over the observation window, or who died prior
to disability incidence, are treated as censored. Using the
Turnbull survival estimates, we derive the cumulative
hazard curve for discrete data (analogous to Kaplan-

Meier curves for continuous data) to illustrate age distri-
butions of onset. Median onset ages are used to deter-
mine a representative ADL loss sequence for each risk
group. Base year individual-level weights are applied in
all analyses to derive a nationally representative sample
and correct for the oversampling of Hispanics, Blacks,
and households in the state of Florida in the survey.
Analyses are conducted using STATA, version 14.0 (Sta-
taCorp, College Station, Tex, USA).

Table 1 Characteristics of subjects in the weighted AHEAD
sample

Variable Mean

Age as at baseline 84.3 (3.56)

Female 60.0%

Years of education 11.3 (3.48)

Marital status:

Not married 7.3%

Married 36.2%

Widowed 56.5%

Prevalence of ADL disabilities
as at wave 2006:

Bathing 34.3%

Dressing 28.4%

Transfer bed /chair 18.3%

Walking 30.3%

Toileting 21.1%

Eating 17.6%

Prevalence of ADL disabilities
as at wave 2014:

Bathing 50.0%

Dressing 51.0%

Transferring bed /chair 33.6%

Walking 42.7%

Toileting 33.6%

Eating 36.1%

Ever have a ‘big four’ chronic
disease (1998–2014)

76.6%

By condition:

Cardiovascular disease (CVD) 59.4%

Cancer 22.0%

Diabetes 14.5%

Chronic lung disease 13.0%

Any of these ‘big four’ conditions 75.0%

Notes: The weighted full sample comprises 4,587,315 subjects (unweighted:
1604). Of these, 3,514,052 subjects (unweighted: 1203) either had a major
chronic disease at 1998 baseline or developed such a condition over the
follow-up period. The comparison group comprises 1,073,263 subjects
(unweighted: 401)

Fong BMC Geriatrics (2019) 19:323 Page 3 of 9

Results
General characteristics of the study population
Table 1 presents the demographic characteristics of the
weighted study population. At the 1998 baseline, the
mean age of the subjects is 84.3 years and 60% are fe-
male. 57% are widowed, 36% are married with spouses
alive, and the rest never married. All subjects began with
no ADL disabilities at baseline. By the 2006 mid-point,
however, many older adults report experiencing diffi-
culty with self-care tasks. About a third of the respon-
dents (34.3%) report difficulty bathing and 30.3% report
difficulty walking. Fewer subjects face difficulty dressing
(28.4%), transferring from bed/chair (18.3%), and eating
(17.6%). Prevalence of ADL disabilities increases over
time. By 2014, the proportions in the weighted sample
having functional limitations are: 50.0% (bathing), 51.0%
(dressing), 33.6% (transferring), 42.7% (walking), 33.6%
(toileting), and 36.1% (eating).
About three-quarters of the subjects or 76.6% had or

developed at least one ‘big four’ chronic disease over
1998–2014. Of the four chronic conditions, CVD is
most prevalent with 59.4% of the weighted sample
reporting that they were ever diagnosed by a doctor to
have stroke, heart attack or heart disease. This is
followed by cancer (22.0%), diabetes (14.5%), and fi-
nally, chronic lung disease (13.0%). Not surprisingly,
mortality is rather substantial among the oldest-old
adults. Approximately 69% of the respondents inter-
viewed at 1998 baseline remained alive as at wave

2002, while 40% of them survived to wave 2006. Over-
all, 92.8% (1488) died over the 16-year the observation
period and 1.8% (29) are lost to follow-up or attrited.
The oldest surviving subject is 104.4 years old at 2014
cut-off.

Disability incidence
Table 2 presents the disability incidence rates for per-
sons with and without major chronic conditions. Cumu-
lative incidence, expressed in percentages, is the number
of new cases of a specific ADL disability (e.g. walking)
observed over 1998–2014 divided by the size of the sub-
population initially at risk. Results show that those with
‘big four’ NCDs are at higher risk of becoming function-
ally impaired. Over the 18-year period, the total inci-
dence rates for bathing, dressing walking, transferring,
toileting, and eating are respectively 44, 41, 42, 29, 31,
and 27% for persons with major NCDs and 36, 33, 32,
25, 25, and 19% for persons without these diseases. It is
also evident that chronically ill persons who become dis-
abled experience higher death rates than their non-ill
disabled counterparts. This holds systematically across
all ADL types. For example, the risk of developing bath-
ing disability and being dead by 2014 is 40.6% for a per-
son with major NCDs as compared to only 32.4% for a
person without such conditions. Although incidence
rates convey information about the risk of becoming dis-
abled, they do not provide insights into the timing or

Table 2 Change in Disability over 18 years (1998 to 2014) among elderly without baseline ADL disabilities

Activity Incident disability by 2014, % No Incident disability by 2014, %

Alive in 2014 Dead by 2014 Total incidence Alive in 2014 Dead by 2014

No major chronic
condition

Bathing 3.5% 32.4% 35.8% 4.0% 60.2%

Dressing 3.8% 29.3% 33.1% 3.6% 63.3%

Walking 3.1% 29.0% 32.1% 4.4% 63.6%

Transferring 2.7% 22.6% 25.3% 4.8% 69.9%

Toileting 3.0% 22.3% 25.3% 4.4% 70.3%

Eating 2.1% 17.1% 19.1% 5.4% 75.5%

Have major chronic
condition

Bathing 3.5% 40.6% 44.1% 1.7% 54.2%

Dressing 3.4% 37.3% 40.7% 1.8% 57.5%

Walking 3.2% 38.4% 41.5% 2.0% 56.4%

Transferring 2.6% 26.7% 29.3% 2.6% 68.1%

Toileting 2.4% 28.3% 30.8% 2.8% 66.4%

Eating 2.5% 24.3% 26.8% 2.8% 70.5%

Notes: Weighted estimates using baseline individual-level weights. The weighted population of the risk group with major chronic condition over 1998–2014
comprises 3,514,052 subjects (unweighted: 1203), while the comparison risk group comprises 1,073,263 subjects (unweighted: 401). Subjects have no baseline
(1998) ADL disabilities and have complete information through 2014 or death

Fong BMC Geriatrics (2019) 19:323 Page 4 of 9

sequence that each disability type occurs over the life
course.

Principal component analysis
Before proceeding to order the ADLs, it is useful to first
ascertain that the six distinct items can be combined to
form an ordering. We perform principal component fac-
tor analysis to investigate the underlying dimensions of
the data. If the responses to the ADL questions are char-
acterized by a single general dimension, then the six
ADLs can be meaningfully combined, otherwise not.
ADLs that are less scalable with other items should be
dropped. The results confirm the existence of a single
general dimension across the ADL items in each survey
wave used. In wave 2006, for instance, the first principal
component explains a high percentage of the total vari-
ance (58.4%); the Cronbach alpha value of 0.86 is also
relatively high indicating reliable estimates. The individ-
ual item-factor loadings on the first component of 0.72–
0.79 are well above the threshold of 0.4 [2].

Age distributions of disability onset
Figure 1 shows the age distribution of onset by activity
for each risk group. The cumulative hazard functions
from the Turnbull analysis are upward sloping implying
that risk of disability onset increases with age. We see a
relatively clear separation of individual curves into two
clusters. Specifically, the bathing, walking, and dressing
curves lies well above the other three curves (toileting,
eating and transferring). In other words, the risk of bath-
ing, walking, and dressing disability onset is considerably
higher as compared to toileting, eating and transferring
disability onset. The small distances between the top
three curves, especially for subjects with major chronic
conditions, suggest there may not be a strong ordering
of these disabilities in the ADL sequence. We note that
the risk of eating disability is generally low for both risk
groups across all ages values examined.
Table 3 presents the derived median ages of disability

onset and interquartile ranges. Results are presented for
the entire sample and by chronic health status (n = 401 no
major chronic condition; n = 1203 have major chronic
condition). For the full sample, the ordered median ages
at disability onset are 91.5 for bathing, 91.8 for dressing,
91.9 for walking, 94.4 for toileting, 94.5 for transferring,
and 95.6 for eating. This yields a representative ADL or-
dering of ‘BDWTPE’ (or bathing, dressing, walking, toilet-
ing, transferring, and eating). These findings mirror the
patterns of disability illustrated in Fig. 1. Specifically, there
is an early loss cluster (‘BDW’) and a late-loss cluster
(‘TPE’). Median onset ages for bathing, dressing, and walk-
ing are extremely close which supports the notion that
there is a weak ordering of these three disabilities for older

American adults. The ordering between toileting and
transferring disabilities is also weak.
Results also indicate that patterns of disability on-

set and functional decline differ by chronic health
status in several ways. First, the activities ordered
separately for each risk group reveal a ‘BDW-TPE’
sequence for persons suffering from major chronic
diseases and a ‘BWD-PTE’ sequence for persons
without such conditions. Second, the median onset
ages are systematically earlier for persons with major
NCDs (range 91.1–95.0) than for those without
(range 93.5–98.1). This holds across all activities and
differences can be substantial. For example, chronic-
ally ill individuals experience difficulty using the toi-
let at age 93.9 on average whereas their counterparts
need help for the same activity only about 37 months
later at age 97.0. For visualization, a graphical com-
parison of the summary ADL orderings is provided
in Fig. 2. There is evidence that disability onset is
compressed within a shorter timeframe for oldest
old adults with major NCDs than for those without.
For the chronically ill, the total estimated gap be-
tween the first and last disability onset is only 3.9
years (compare 4.6 years for those without major
chronic conditions).
We also conducted additional analyses stratified by

gender. Comparing females with and without major
chronic diseases, for example, we find that the patterns
of disability for each subgroup reveal an early loss clus-
ter and a late-loss cluster (although the exact sequence
of the ADLs may vary between subgroups). The earlier
finding that median onset ages are systematically earlier
for those with major NCDs also hold when the analyses
is conducted separately by gender. For instance, median
ADL onset ages for chronically ill females are 90.3–94.0
as compared to 92.6–98.6 for females without the dis-
eases. Consequently, functional decline occurs at a more
rapid pace and is more compressed among females (and
separately, males) with major NCDs as compared to
their same-sex peers.

Disability and functional loss are not static constructs in
old age. This paper presents new evidence on ADL dis-
ability incidence and ensuing patterns of disability pro-
gression in a nationally representative sample of older
Americans aged 80 and above. We exploit panel data
over an 18-year period to paint a mathematical picture
of functional decline among the oldest-old as they ad-
vance in age. We also explore the nexus between disabil-
ity and chronic illnesses. This study is the first, to our
knowledge, that longitudinally evaluates and compares
ADL loss sequences for older adults with and without

Fong BMC Geriatrics (2019) 19:323 Page 5 of 9

major chronic conditions. Three important findings, in
particular, deserve comment.
First, our findings indicate that older adults who ever

have any of the ‘big four’ NCDs are at higher risk of be-
coming functionally disabled than persons without such
diseases. Disability incidence rates, across all ADL
items, are higher for persons with major chronic dis-
eases than for persons without such conditions. A
widely-held assumption is that persons with major
NCDs generally face higher risk of mortality. Our ana-
lysis reveals that it is critical to distinguish between
persons with incident disability versus those without
disability in this aspect. Specifically, we observe that

only chronically ill older adults who are also function-
ally impaired are exposed to greater risk of death. The
proportions of non-disabled older adults who died dur-
ing the observation period is comparable between both
subgroups.
Second, we show that disability onset is systematic-

ally earlier for older adults with major NCDs. In
addition, and importantly, that their disability pro-
gression is compressed within a shorter timeframe.
For the chronically ill, multiple disabilities strike al-
most at the same time and gaps between the onset of
one disability and the next is small. In other words,
this risk group face a steeper slope of functional

Fig. 1 Age distributions of onset by ADL disability. Panel a Subjects with major chronic conditions. Panel b Subjects without major chronic
conditions. Notes: Weighted estimates using baseline individual-level weights. The weighted population of the risk group with major chronic
condition over 1998–2014 comprises 3,514,052 subjects (unweighted: 1203), while the comparison risk group comprises 1,073,263 subjects
(unweighted: 401).

Fong BMC Geriatrics (2019) 19:323 Page 6 of 9

decline as compared to their counterparts. This has
profound implications. Chronic care delivery pro-
grams that seek to offer higher quality of care need
to take into account that older patients may experi-
ence a loss of function or worsening of functional
capabilities during their period of care, and care hours
have to be changed accordingly to adapt to such dynamic
realities. As a patient becomes afflicted with more ADL
disabilities, chronic care can become more complex and
expensive. This underscores the importance of consistent
care for chronically ill persons for whom an interruption
in care can lead to exacerbation, or even death.
Third, our analyses are informative on how ADL loss

sequences compare between older Americans with and
without major NCDs. We find two broad similarities.
Regardless of chronic health status, the progression of
functional loss is characterized by an early loss cluster

and a late-loss cluster. The former comprises bathing,
dressing, and walking, while the latter comprises toilet-
ing, transferring, and eating (items listed in no particular
order). This separation of item clusters is consistent the
finding in Fong & Feng [12] based on the Rasch scaling
model. Another similarity is that bathing disability oc-
curs first and the eating disability last in both risk
groups – a finding that concurs with the ADL hierarch-
ies derived in previous studies for geriatric populations
in the U.S. and elsewhere [4–6, 10, 12]. One difference,
however, in the two representative ADL orderings is that
chronically ill persons are likely to lose functional cap-
acities in a ‘BDW-TPE’ sequence whereas their counter-
parts tend to do so in a ‘BWD-PTE’ sequence. This
subtle difference can be rationalized in part by the weak
orderings observed in the early-loss cluster disabilities,
and separately in the toileting and transferring

Fig. 2 Onset age of ADL disabilities for those with and without major chronic conditions. Notes: The weighted population of the risk group with
major chronic condition over 1998–2014 comprises 3,514,052 subjects (unweighted: 1203), while the comparison risk group comprises 1,073,263
subjects (unweighted: 401)

Table 3 Median Age of Onset by ADL Disability

All (N = 1604) No major chronic
condition (n = 401)

Have major chronic
condition (n = 1203)

ADL disability Median (25, 75%)a Median (25, 75%) Median (25, 75%)

Bathe (B) 91.5 (87.2, 95.9) 93.5 (102.2, 97.0) 91.1 (86.8, 95.5)

Dress (D) 91.8 (87.4, 96.6) 93.9 (155.4, 97.9) 91.4 (86.9, 96.0)

Walk (W) 91.9 (87.5, 96.8) 93.6 (88.3, 159.5) 91.5 (87.3, 96.1)

Toilet (T) 94.4 (89.2, 100.0) 97.0 (92.0, 101.2) 93.9 (88.8, 101.2)

Transfer (P) 94.5 (89.7, 99.9) 96.6 (90.8, 101.0) 94.2 (89.4, 100.9)

Eat (E) 95.6 (90.4, 101.1) 98.1 (93.0, 101.4) 95.0 (90.2, 99.7)
a25, 75% = interquartile range
Notes: The ADL disabilities are presented in ascending order of their median ages of disability onset (rounded to one decimal place). Weighted estimates using
baseline individual-level weights. The weighted population of the risk group with major chronic condition over 1998–2014 comprises 3,514,052 subjects
(unweighted: 1203), while the comparison risk group comprises 1,073,263 subjects (unweighted: 401). Subjects have no baseline (1998) ADL disabilities and have
complete information through 2014 or death

Fong BMC Geriatrics (2019) 19:323 Page 7 of 9

disabilities, in our full sample as well as in some prior
studies [3, 10].

Conclusions
Our study of disability emphasizes that prevention of
functional decline should target major noncommunicable
diseases in older adults. In addition, disease management
programs for chronically ill older adults should take a
closer look at new interventions to help patients delay dis-
ability onset and optimize functional autonomy within
emerging models of chronic care. The small gaps in onset
ages within the cluster of early-loss disabilities is particu-
larly worrisome as this suggests that these three disabilities
tend to strike together. Consequently, older Americans
and especially those with major chronic conditions who
have difficulty with any one of these disabilities (e.g. bath-
ing) are at high risk of developing the other two disabil-
ities. Dependency in three or more ADLs, in turn, is
associated with the need for long-term care and adverse
outcomes such as nursing home admission [23, 25, 26].
This study has limitations. First, the AHEAD measures

of ADLs are self-reported, yet normative perceptions of
“having difficulty” with a particular task may vary across
individual respondents. For example, some studies con-
tend that individuals are more likely to report the having
difficulty with self-care tasks if they have access to care-
givers [27]. Second, sample shrinkage over the follow-up
period is another limitation. Mortality tends to be a
problem in most studies focusing on the oldest adults,
and in our case, the low rate of attrition or being lost to
follow-up (1.8%) provides some reassurance. Future re-
search using richer longitudinal data can investigate fur-
ther how patterns in disability onset vary by specific
diseases and whether such patterns are modifiable de-
pending on individuals’ health behaviour, social supports
and other factors in the environment. Further work is
also needed to develop prevention strategies to delay on-
set of ADL disabilities and interventions to meet the
needs of older people as these disabilities occur.

ADL: Activities of daily living; AHEAD: Asset and Health Dynamics of the
Oldest Old; CVD: Cardiovascular diseases; IRT: Item response theory;
NCD: Noncommunicable disease

The author is grateful to Qiushi Feng, Kazuhiro Harada, and Jeong-Hwa Ho
for their helpful comments and suggestions.

JF designed the study and was responsible for the collection, analysis, and
interpretation of data, as well as writing the manuscript. The author read and
approved the final manuscript.

The research was supported by the Singapore Ministry of Education Start-up
Grant at the National University of Singapore. The funding body did not in-
fluence this paper in any way prior to circulation.

The datasets analysed in the current study are publicly available in the
Health and Retirement Study repository. The data products are available
without cost to registered users. More information can be found at (http://
hrsonline.isr.umich.edu).

Not applicable.

Not applicable.

The author declares that she has no competing interests.

Received: 20 May 2019 Accepted: 6 November 2019

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Abstract
Background
Methods
Results
Conclusions
Background
Methods
Data and measures
Statistical analyses
Results
General characteristics of the study population
Disability incidence
Principal component analysis
Age distributions of disability onset
Discussion
Conclusions
Abbreviations
Acknowledgements
Author’s contributions
Funding
Availability of data and materials
Ethics approval and consent to participate
Consent for publication
Competing interests
References
Publisher’s Note