The impact of disease-related impairments on disability and health-related quality of life: a systematic review

BMC Medical Research Methodology BioMed Central Research article Open Access The impact of disease-related impairments on disability and health-related quality of life: a systematic review Nadine Weisscher*1, Rob J de Haan2 and Marinus Vermeulen1 Address: 1Department of Neurology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands and 2Department of Clinical Epidemiology and Biostatistics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands Email: Nadine Weisscher* - ; Rob J de Haan - ; Marinus Vermeulen - * Corresponding author Published: 19 June 2007 BMC Medical Research Methodology 2007, 7:24 doi:10.1186/1471-2288-7-24 Received: 17 January 2007 Accepted: 19 June 2007 This article is available from: http://www.biomedcentral.com/1471-2288/7/24 © 2007 Weisscher et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: To investigate the interchangeability of measures of disability and health-related quality of life (HRQL) by comparing their associations patterns with disease-related impairment measures in patients with a variety of conditions. Methods: A systematic literature search of MEDLINE, EMBASE, Web of Science and a hand search of reference lists through January 2006. Studies were included if they reported associations patterns between impairment and disability and between impairment and HRQL. Correlation coefficients were transformed to Fisher's z effect size (ES(z)). Weighted averages were reported as pooled ES(z) with 95% confidence intervals (CI). Results: The relationship between impairment and disability was stronger (pooled ES(z) = 0.69; 95% CI, 0.66 – 0.72) than between impairment and HRQL (pooled ES(z) = 0.38; 95% CI, 0.36 – 0.41). The physical component score (pooled ES(z) = 0.43; 95% CI, 0.39 – 0.47) and disease-specific HRQL (pooled ES(z) = 0.46; 95% CI, 0.40 – 0.51) were stronger associated with impairments than the mental component score (pooled ES(z) = 0.28; 95% CI, 0.20 – 0.36) and generic HRQL (pooled ES(z) = 0.36; 95% CI, 0.33 – 0.39). Conclusion: This study shows measures of disability and different HRQL domains were not equally related to impairment. Patient's impairments are better reflected in disability measures, than in HRQL instruments. There are many outcomes of interest and precisely defining them and measuring them will improve assessing the impact of new interventions. Background Choosing an outcome measure for use in clinical research is a complex process. If the outcomes are chosen inappropriately, a study may provide unreliable results [1]. Frequently used outcomes measures are mortality rates, number of events (recurrent myocard infarction) or disease activity (lesion load on MRI). Besides biological measures other levels of clinical measurement can be considered in clinical studies: impairments, disability and health-related quality of life (HRQL). Impairments are the direct organic manifestations of the disease such as consciousness and paresis [2,3]. Disability can be defined as limitations in carrying out activities of daily living, such as self-care, mobility and activities inside or outside the home [2,3]. HRQL refers to Page 1 of 10 (page number not for citation purposes) BMC Medical Research Methodology 2007, 7:24 a broad spectrum of consequences of disease. Although this concept also includes elements of impairments and disabilities, it has a strong focus on patient's social functioning and perceived health status and well being [4,5]. Hence, impairment measures are closely related to the patient's disorder, whereas the other outcome measures focus on the patient's level of functional health. Each subsequent level of clinical measurement is increasingly less disease-specific and more relevant to the patient. Recently, patient-relevant outcomes in terms of disability and HRQL have become more important [6]. Although both functional outcomes reflect the consequences of diseases on personal level, they are conceptually different and not synonymous as is often thought by clinical researchers. Consequently, the decision to use one of these measures can have important implications for the interpretation of the study results. The objective of this systematic review is to investigate the interchangeability of measures of disability and HRQL by comparing their association patterns with disease-related impairment measures. Methods Data sources We conducted a systematic literature search of MEDLINE from 1966 through January 2006, EMBASE from 1980 through January 2006 and Web of Science from 1988 through January 2006 to identify studies addressing the relation between outcome measures. A search strategy using Medical Subject Heading, text words and Publication Types Impairment or Body function (Publication date from 2001) or Body structure (Publication date from 2001) and Disability (Evaluation) or Activity or Disabled Persons or Activities of Daily Living and Health-related quality of life combined with Association or Evaluation Studies or Comparative Study or Validation Studies was used. Studies from English, German and French literature were included. The electronic search was supplemented by hand searching the investigators files, and retrieval of references cited in available literature. The search strategy was composed by one of the authors (NW) in consultation with a clinical librarian. Study selection A set of explicit criteria, composed by three investigators (NW, RdH, MV), were used for selection of the literature. Articles were included if they focused on methodological or metric aspects of patient-based outcomes (for example, methods of evaluating such measures, psychometric or clinimetric assessment of measures, comparative studies of measures). Studies were included in which both the association between impairment and disability, and between impair- http://www.biomedcentral.com/1471-2288/7/24 ment and HRQL were calculated by means of correlation coefficients or other association measures, regardless of the disease with the exception of psychiatric disorders. Hence, to improve the comparability between the correlation patterns between the different health concepts, only studies were included in which the three types of health outcomes were assessed in the same patient population. The measurement instruments were questionnaires or observation lists. When a multi-scale questionnaire focuses on more than one concept we only included the sub-scale for the health domain in question. In case of double publication we selected the first publication in time. Excluded were studies which valuated HRQL in terms of utilities [7] or composite scores of conceptual different measures. Data extracti (...truncated)