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Functional status, life-space mobility, and quality of life: a longitudinal mediation analysis

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Abstract

Purpose

Using the Wilson–Cleary model of patient outcomes as a conceptual framework, the impact of functional status on health-related quality of life (HRQoL) among older adults was examined, including tests of the mediation provided by life-space mobility.

Methods

Participants were enrollees in a population-based, longitudinal study of mobility among community-dwelling older adults. Data from four waves of the study equally spaced approximately 18 months apart (baseline, 18, 36, and 54 months) were used for participants who survived at least 1 year beyond the 54-month assessment (n = 677). Autoregressive mediation models using longitudinal data and cross-sectional mediation models using baseline data were evaluated and compared using structural equation modeling.

Results

The longitudinal autoregressive models supported the mediating role of life-space mobility and suggested that this effect is larger for the mental component summary score than the physical component summary score of the SF-12. Evidence for a reciprocal relationship over time between functional status, measured by ADL difficulty, and life-space mobility was suggested by modification indices; these model elaborations did not alter the substantive meaning of the mediation effects. Mediated effect estimates from longitudinal autoregressive models were generally larger than those from cross-sectional models, suggesting that mediating relationships would have been missed or were potentially underestimated in cross-sectional models.

Conclusions

These results support a mediating role for life-space mobility in the relationship between functional status and HRQoL. Functional status limitations might cause diminished HRQoL in part by limiting mobility. Mobility limitations may precede functional status limitations in addition to being a consequence thereof.

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Notes

  1. Separate models were fit for PCS and MCS primarily for the ease of presentation of results and because each variable was considered a separate end point. It is possible to estimate one complete model containing both PCS and MCS. Given that the PCS and MCS were originally constructed to be orthogonal, there should be little difference in the results based on separate models and results from a single model considering both PCS and MCS. Indeed, when considering the final models reported in the current analysis, there were no substantive differences in findings when comparing separate estimation models for PCS and MCS and a single, combined model.

  2. The nature of the calculation of the LSC score limits the applicability of internal consistency reliability and thus Cronbach’s alpha is not reported for the LSC; as stated earlier, past research reports high levels of test–retest reliability for this measure.

  3. As suggested by one anonymous reviewer, in addition to examining chi-square change and model fit statistics for assessing measurement invariance, one should consider whether any meaningful differences in the estimates of interest occur between models with constrained invariant parameters and those where parameters are freely estimated. In the present study, the mediated effects were highly similar between models with measurement invariance for functional status and those with non-invariance. For example, the total mediated effect estimate for the PCS Model 2 assuming invariant thresholds and loadings (see Table 2) was −0.88 (p < 0.0001); this same effect was −0.90 (p < 0.0001) in a model with non-invariance. An examination of the time-specific indirect effects revealed similar results; p values for these indirect effects differed in the third decimal place, if at all.

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Acknowledgments

The UAB Study of Aging is funded through a grant from the National Institute on Aging (R01 AG015062).

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Authors and Affiliations

  1. School of Pharmacy, University of Mississippi, Faser Hall 235, University, MS, 38677, USA

    John P. Bentley

  2. Birmingham VA Medical Center, University of Alabama at Birmingham, CH19 Suite 201, 1720 2nd Avenue South, Birmingham, AL, 35294-2041, USA

    Cynthia J. Brown

  3. Division of Gerontology, Geriatrics, and Palliative Care, University of Alabama at Birmingham, CH19 Suite 201, 1720 2nd Avenue South, Birmingham, AL, 35294-2041, USA

    Cynthia J. Brown & Patricia Sawyer

  4. Department of Epidemiology, University of Alabama at Birmingham, EFH 609, 1720 2nd Avenue South, Birmingham, AL, 35294-2041, USA

    Gerald McGwin Jr.

  5. Atlanta VA Geriatric Research, Education, and Clinical Center, Center for Aging, and Division of Gerontology, Geriatrics, and Palliative Care, University of Alabama at Birmingham, CH19 Room 201, 1720 2nd Avenue South, Birmingham, AL, 35294-2041, USA

    Richard M. Allman

  6. Center on Aging and Health, Johns Hopkins University, 2024 E. Monument Street, Suite 2-700, Baltimore, MD, 21205, USA

    David L. Roth

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  1. John P. Bentley
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Correspondence to John P. Bentley.

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Bentley, J.P., Brown, C.J., McGwin, G. et al. Functional status, life-space mobility, and quality of life: a longitudinal mediation analysis. Qual Life Res 22, 1621–1632 (2013). https://doi.org/10.1007/s11136-012-0315-3

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