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

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.