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Effect of Subclinical Status in Functional Limitation and Disability on Adverse Health Outcomes 3 Years Later

¹ Iowa City VAMC, Iowa.
² Indiana University, Indianapolis.
³ Regenstrief Institute, Inc., Indianapolis, Indiana.
⁴ Gainesville VAMC, Florida.
⁵ University of Florida, Gainesville.
⁶ Saint Louis University, Missouri.
⁷ Washington University in St. Louis, Missouri.
⁸ The University of Iowa, Iowa City.

Address correspondence to Fredric D. Wolinsky, PhD, The John W. Colloton Chair in Health Management and Policy, College of Public Health, The University of Iowa, 200 Hawkins Dr., E-205 General Hospital, Iowa City, IA 52242. E-mail: fredric-wolinsky{at}uiowa.edu

	Abstract

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Background. This article examines the effect of self-reported, baseline subclinical status (i.e., independent but adaptive performance) for functional limitation and disability on adverse health outcomes.

Methods. Nine hundred ninety-eight African-American men and women aged 49–65 years received in-home evaluations at baseline, and 853 were re-evaluated 3 years later. Baseline subclinical status was ascertained for five lower body tasks and seven activities of daily living (ADLs)/instrumental ADLs (IADLs). Outcomes included difficulty with lower body limitations, ADLs/IADLs, physical performance, physician visits, hospitalization, nursing home placement, and mortality.

Results. The baseline proportion of subclinical status evidence for the five lower body items was 0.33 (standard deviation [SD] = 0.20), and for the seven ADLs/IADLs was 0.20 (SD = 0.30). Significant independent effects of subclinical status for lower body limitations were observed on physician visits and hospitalization. Significant independent effects of subclinical status for ADLs/IADLs were observed on ADLs/IADLs and physician visits.

Conclusions. Subclinical status for functional limitation and disability independently predicts several subsequent adverse health outcomes, although the effects of the latter (ADLs/IADLs) are stronger. Interventions to reduce frailty should focus on self-reported subclinical status as an early warning system.

Ascertaining subclinical status involves asking two additional questions for each functional limitation, activity of daily living (ADL), or instrumental ADL (IADL) task. One focuses on modifying the method of task performance (e.g., "Because of health or physical problems, have you changed the method you use to bathe or shower?"). The other focuses on modifying the frequency of task performance (e.g., "Because of health or physical problems, do you bathe or shower less often now?"). This approach facilitates the identification of respondents who report no difficulty with task performance, but who have compensated for their functional decrement by modifying either the method or frequency of their task performance.

Using cross-sectional data, Fried and colleagues (2) have shown considerable support for the reliability and construct validity of the subclinical status concept. Furthermore, longitudinal analyses of data from the Women's Health and Aging Study (WHAS) have demonstrated its predictive validity in terms of incident disability (4,5). In data on 436 community-dwelling women who were 70–80 years old at baseline, subclinical status in two tasks—walking one half mile or more and climbing up 10 steps—were significant predictors of incident disability in those tasks 18 months later.

Recently, further support for the reliability and contextual validity of subclinical status measures for both functional limitation and disability were provided by data from the 998 respondents in the African American Health (AAH) study (5,6). Those reports showed that subclinical status measures at baseline were robust predictors of the onset of the respective functional limitation or disability within 2 years (7). No reports in the literature, however, either demonstrate more enduring effects of subclinical status or provide evidence of its predictive utility on adverse health outcomes other than functional limitations and disability.

This article does just that. Using data from the AAH study, the extent of subclinical status at baseline is determined for five lower body functions and seven ADL/IADL tasks. The extent of subclinical status for each of these two sets of tasks at baseline is then used to predict adverse outcomes occurring 3 years later. The adverse outcomes include difficulty with lower body functions, difficulty with ADLs/IADLs, physical performance, physician visits, hospitalization, nursing home placement, and mortality. To determine the unique contribution of subclinical status in predicting these adverse health outcomes, two sets of models are estimated. The first set adjusts for demographic, socioeconomic, and comorbidity factors. The second set adjusts for those factors as well as the aggregate level of baseline lower body function and ADL/IADL difficulties. To examine the potential for modification of the subclinical status effects on these outcomes, sensitivity analyses are used to replicate the first set of models within three strata based on the aggregate level of baseline lower body function and ADL/IADL difficulties.

	METHODS

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Sample
The sampling design of the AAH has been described elsewhere (5,6). The AAH includes 998 African Americans who were born from 1936 through 1950 and who lived in a poor, inner-city area previously studied by the investigative team (8), or the near northwest suburbs. To recruit equal numbers of participants from both strata, unequal sampling proportions were set; therefore, analysis of probability-weighted data is reported. Beyond age, inclusion criteria involved the ability and willingness to sign informed consent, self-reported black or African American race, and Mini-Mental State Examination (MMSE) score 16 (only 15 participants scored < 20) (9). Baseline evaluations averaged 2.5 hours and occurred in the participant's home between September 2000 and July 2001. The response rate was 76%. Thirty-six-month follow-up evaluations also occurred in the participant's home and averaged 2.0 hours. Of the 998 original participants, 853 were successfully reinterviewed at 36 months, and there were 51 deaths. Thus, the reinterview rate for surviving participants was 90%.

Subclinical Status
Participants were asked a comprehensive series of standard lower body function, ADL, and IADL questions. If the participant expressed any difficulty or was unable to perform the function or task, s/he was considered to be limited (or have disability) in that lower body function, ADL, or IADL item. If the participant reported having no difficulty in performing the targeted function or task, the two follow-on subclinical status questions [method and frequency (1)] were asked. Subclinical status was assessed for five lower body functions (walking one-half mile, climbing steps, stooping/crouching/kneeling, using fingers to grasp or handle, and lifting or carrying something as heavy as 10 pounds) and seven ADLs/IADLs (bathing and showering, dressing, getting in and out of bed and chairs, preparing meals, doing light housework, doing heavy housework, and managing medications). Participants reporting no task difficulty, but who changed the method or decreased the frequency of performing it (or both), were defined as having subclinical status on that item.

Construction of a summary measure across the subclinical status items is a bit complicated because a simple count of the lower body or the ADL/IADL items for which the participant reported subclinical status must be adjusted for the number of status assessments that were made. Therefore, the proportion of assessments for which subclinical status was reported in each of these two domains is used. This proportion is determined by dividing the number of tasks for which the participant reported changing the method or frequency of performance by the number of tasks assessed (of a maximum of five for lower body function and seven for ADLs/IADLs). For example, if a participant reported difficulty for three of the seven ADLs/IADLs and changed the method or frequency of performance for two of the remaining four ADLs/IADLs, the proportion of subclinical status evidence was 0.50. These proportions are then multiplied by 10, so that the regression coefficients and odds ratios (ORs) reflect the effect associated with a 10% increase in the extent of subclinical status.

Adverse Outcomes
Seven adverse outcomes were measured: lower body limitations, difficulty with ADLs/IADLs, physical performance, physician visits, hospitalization, nursing home placement, and mortality. Lower body limitations were measured as the sum of five items (walking one-quarter mile, going up and down one flight of steps, stooping/crouching/kneeling, lifting 10 pounds, and pushing large objects) for which the participant reported difficulty. Similarly, ADLs/IADLs were measured as the sum of seven items (bathing, dressing, getting in and out of a chair, preparing meals, doing light housework, doing heavy housework, and managing medications) for which the participant reported difficulty. Physical performance was measured using the 0–12 Short Physical Performance Battery (SPPB) summary score developed by Guralnik and colleagues (10). The SPPB summary score is based on three components: gait speed (m/s), five repeated chair stands (s), and tandem stand with eyes open (s). Each component is scored from 0 to 4, with 0 indicating that the participant was unable or unwilling to perform, and the four remaining categories reflecting approximate quartiles in the baseline distributions. Physician visits were measured as the self-reported number that occurred in the year prior to baseline and in the year prior to the 36-month follow-up interview. The hospitalization measure reflects whether the participant was hospitalized in the year prior to follow-up. The two remaining outcomes reflect whether the participant was placed in a nursing home or died by the time of the 36-month follow-up.

Covariates
Numerous risk factors have been identified for ADLs/IADLs, lower body limitations, health services use, and mortality (11–14). Accordingly, several demographic, socioeconomic, and comorbidity measures were included to determine the independent effect of subclinical status in lower body function and ADLs/IADLs at baseline on the targeted adverse outcomes (5–7). Demographic characteristics included age, sex, and marital status. The socioeconomic status indicators were education and perceived income adequacy. Comorbidity was measured using markers for diabetes, cancer, heart disease, lung disease, elevated systolic or diastolic blood pressure, clinically relevant levels of depressive symptoms, and cognitive ability, using the MMSE.

In addition, to ensure that the unique effect of subclinical status is identified, the second set of models also adjusts for baseline levels of difficulties with lower body function and ADLs/IADLs. This adjustment is made by first determining the aggregate number of these limitations and then creating a set of two dummy variables reflecting that distribution. The two dummy variables reflect low (1–3) and high (4 or more) levels of baseline difficulties, with the reference or omitted category being those participants who reported no such limitations at baseline.

Analytic Models
Because lower body limitations, ADLs/IADLs, the SPPB, and physician visits are essentially count variables for which the traditional assumptions of multiple linear regression do not hold, residualized change score (15) multiple Poisson regression techniques were used to model these outcomes. In these models, the 36-month follow-up value of the outcome is initially regressed on its baseline value and the extent of subclinical status evidence for the lower body function and ADL/IADL domains. This step provides estimates of the crude effects of subclinical status in these two domains on changes in the outcome measure. The covariates (and, in the second set of models, the dummy variables reflecting the aggregate distribution of lower body function and ADL/IADL difficulties) are then added to these models to obtain the independent effects of the extent of subclinical status evidence in these two domains on the adverse outcomes. For all binary outcomes, similar multiple logistic regression models were used with the same sequential introduction of the covariates (16). Sensitivity analyses were conducted replicating the first set of adjusted models within each of three strata based on the aggregate level of baseline lower body function and ADL/IADL difficulties to examine the potential for modification of the subclinical status effects on these outcomes.

	RESULTS

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Sample Characteristics
At baseline, the mean age of the 998 participants was 56.8 years, 42% were men, 47% were currently married, 28% were divorced or separated, 13% were widowed, and 12% were single. The mean educational attainment was 12.5 years, and 15% reported not having enough income to make ends meet. Twenty-six percent had diabetes, 7% had cancer, 10% had heart disease, 5% had lung disease, 21% had clinically relevant levels of depressive symptoms, 49% had elevated systolic and 32% had elevated diastolic blood pressure, and the mean score on the MMSE was 27.9. The mean number of lower body difficulties was 1.33 (standard deviation [SD] = 1.59; 46.5% had none), the mean number of ADLs/IADLs was 0.80 (SD = 1.46; 67.1% had none), 44% had no lower body or ADL/IADL difficulties, 31% had 1–3, and 25% had 4 or more. The mean SPPB summary score was 8.06 (SD = 3.31), and the mean number of physician visits was 4.35 (SD = 3.85).

The mean baseline proportions of the two domains of subclinical status was 0.33 (SD =.38) for lower body limitations, and 0.20 (SD =.30) for ADLs/IADLs. Figures 1 and 2 contain histograms of these distributions, which emphasize the substantial modal response at 0%, and the generally uniform distribution thereafter, with the exception of the typical heaping of count measures at the upper extremes. Note that, in the models estimated, these proportions are multiplied by 10 (resulting in means of 3.3 and 2.0 for lower body limitations and ADLs/IADLs, respectively) so that the effects will be calibrated as those associated with an additional 10% evidence of subclinical status. At the time of the 36-month follow-up, the mean number of lower body limitations was 1.45, the mean number of ADLs/IADLs was 0.94, the mean SPPB summary score was 8.45, the mean number of physician visits was 4.65, 164 participants (16%) had been hospitalized, 8 participants (0.8%) had been placed in nursing homes, and 51 participants (5.1%) had died.

View larger version (5K): [in this window] [in a new window]   Figure 1. Number of participants at each proportional level of evidence of subclinical status at baseline on the five lower body functions

View larger version (5K): [in this window] [in a new window]   Figure 2. Number of participants at each proportional level of evidence of subclinical status at baseline on the seven activities of daily living/instrumental activities of daily living

Multiple Logistic Regression Analyses
Table 2 contains the crude and adjusted (Models 1 and 2) ORs obtained from multiple logistic regression of whether the participant was hospitalized in the year prior to the 36-month follow-up interview, was placed in a nursing home, or died within 3 years of baseline. Hospitalization and nursing home placement were evaluated for the 853 participants re-interviewed at the 36-month follow-up, whereas mortality was evaluated for all 998 participants. Because the proportion of subclinical status evidence for each domain has been multiplied by 10, these ORs can be interpreted as reflecting the comparison of risk associated with having a 10% increment in subclinical status evidence on the assessed tasks in the given domain. As shown, the only independent significant effect observed was that of baseline subclinical status in lower body function on subsequent hospitalization, although the effect of subclinical status in ADLs/IADLs was marginally insignificant. These two effects are of similar magnitude, although subclinical status in ADLs/IADLs increases the risk of subsequent hospitalization, whereas subclinical status in lower body function decreases the risk of subsequent hospitalization.

	DISCUSSION

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The contributions to the literature in this article involve extensions in (i) the follow-up period over which the effect of subclinical status has been demonstrated, and (ii) the range of adverse outcomes affected by subclinical status. In terms of the former, previous studies by Fried and colleagues (4,17) and Wolinsky and colleagues (7) reported that subclinical status was a robust predictor of selected functional limitations and disability at 18 and 24 months, respectively. The results presented here extend the reach of the effects of subclinical status to 36 months. In terms of the range of affected outcomes, previous reports by Fried and colleagues (4,17) and Wolinsky and colleagues (7) were limited to four functional limitations (walking one half mile, going up and down one flight of stairs, stooping/crouching/kneeling, and lifting or carrying 10 pounds) and one ADL item (doing heavy housework). The results presented here extend the range of adverse outcomes for which statistically significant independent effects of subclinical status have been identified to five lower body functional limitations, seven ADLs/IADLs, the number of physician visits, and hospitalization. Thus, the effects of subclinical status on subsequent adverse health outcomes are more enduring and widespread than previously reported.

The magnitudes of these effects, however, are modest. Indeed, the maximal contrast would involve comparing a participant with 0% subclinical status evidence in either domain versus one with 100% subclinical status evidence, which would be equivalent to 10 times the adjusted Poisson regression estimates shown in Table 1 (which are calibrated at a 10% difference comparison). For the lower body function and ADL/IADL outcomes, the maximal contrast represents an increase of only one fifth to one half of an additional lower body or ADL/IADL limitation over 3 years. That amount, however, is approximately similar in magnitude to the baseline prevalence for these outcomes, and thus should not be considered trivial. Similarly, the same maximal contrast for the subclinical status in ADLs/IADLs translates into 0.8 more physician visits and a 74% increase in the likelihood of being hospitalized. Thus, the main effect of subclinical status in disability appears to involve future demands on the health care delivery system, most likely in an attempt to stabilize the disablement process.

Before discussing the implications of these results, four issues warrant attention. First, these results were obtained from a probability-based, representative community sample of African American men and women who were 49–65 years old at the time of their baseline interviews (5–7). In contrast, the pioneering works of Fried and colleagues (4,17) were based on older adults, most of whom were white women who enjoyed high levels of physical and cognitive abilities. This raises the question of whether the results reported here generalize to other middle-aged minority and majority groups. Of course, further research using more diverse samples is needed to definitively resolve that question. What is more likely, however, is that given the earlier onset and increased burden of functional limitation, disability, and resultant health services use among disadvantaged minorities (18–20), the more enduring and widespread effects of subclinical status in functional limitation and disability was simply more palpable in this sample of middle-aged African American men and women.

Second, no independent effects of subclinical status in functional limitations or disability were observed on nursing home placement and mortality. This finding is most likely due to the limited statistical power for these outcomes. Indeed, only 8 participants were placed in a nursing home during the 3-year follow-up window, and only 51 participants died during that time. To avoid over-fitting event history models, it is generally recommended that there be 10 events (nursing home placements or deaths) for each predictive term in the logistic regression equation (21–23). These data do not meet that minimum, and this likely explains why the significant crude effect of subclinical status for ADLs/IADLs on mortality is fully decomposed by the introduction of the covariates.

Third, the effects of subclinical status in the ADL/IADL domain are stronger, more widespread, and consistent with expectations compared to the less robust and selective effects of subclinical status in the functional limitation domain. This is understandable. In traditional models of the disablement process, the primary pathway is from impairments to functional limitations to disability to subsequent outcomes (3). Thus, the effects of subclinical status in the functional limitation domain are subject to mediation by those in the disability domain.

Finally, it should be noted that the effects of subclinical status in lower body limitations and ADLs/IADLs on physician visits and hospitalization are in the opposite direction. Indeed, the former appear to be counterintuitive, whereas the latter are more consistent with general expectations. Two plausible explanations may help to reconcile this apparent anomaly. On the one hand, subclinical status in lower body function may serve as an access barrier for physician visits, and less frequent physician use may reduce the risk of hospitalization. On the other hand, the onset of (or increases in) difficulties with ADLs (especially those that are more cognitively demanding) initially increases the demand for health services (reflecting diagnostic and evaluation activity), followed by a triage and selection process (associated with forgoing subsequent treatment) after recovery is deemed unlikely (24–30). In this study of middle-aged adults, it is unlikely that the initial increase in service demand has given way to the triage and selection processes. Further evaluation of these explanations, however, is beyond the scope of this article.

These issues notwithstanding, the results reported here have important implications for health policy. To reduce frailty, interventions should focus on self-reported subclinical status as "an early warning system." This is entirely consistent with the recent Institute of Medicine report (31) that distinctly targets as a national priority the reduction of frailty among older adults. Inasmuch as existing interventions focus on difficulty (rather than subclinical status), their success may be minimized because their target is recovery rather than prevention. Thus, given the relative ease of ascertaining subclinical status and its ability to predict the onset of multiple adverse outcomes, health policy should seize the opportunity to move the focus on interventions further upstream in the disablement process to more effectively address prevention rather than recovery.

	Acknowledgments

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This work was supported in part by National Institutes of Health Grant R01 AG-10436 to Douglas K. Miller. Dr. Wolinsky is Associate Director of the Center for Research in the Implementation of Innovative Strategies in Practice (CRIISP) at the Iowa City VA Medical Center, and Dr. Andresen is a Core Investigator with the Rehabilitations Outcomes Research Center (RORC) at the North Florida/South Georgia Veterans Health System, both of which are funded through the Department of Veterans Affairs, Veterans Health Administration, Health Services Research and Development Service.

	Footnotes

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Decision Editor: Darryl Wieland, PhD, MPH

Received September 19, 2005

Accepted May 8, 2006

	References

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