Empirical Validity of a Generic, Preference-Based Capability Wellbeing Instrument (ICECAP-A) in the Context of Spinal Cord Injury

Background Assessing the validity of generic instruments across different clinical contexts is an important area of methodological research in economic evaluation and outcomes measurement. Objective Our objective was to examine the empirical validity of a generic, preference-based capability wellbeing instrument (ICECAP-A) in the context of spinal cord injury. Methods This study consisted of a secondary analysis of data collected using an online cross-sectional survey. The survey included questions regarding demographics, injury classifications and characteristics, secondary health conditions, quality of life and wellbeing, and functioning in activities of daily living. Analysis comprised the descriptive assessment of Spearman's rank correlations between item-/dimension-level data for the ICECAP-A and four preference-based health-related quality of life (HRQoL) instruments, and discriminant and convergent validity approaches to examine 21 evidence-informed or theoretically derived constructs. Constructs were defined using participant and injury characteristics and responses to a range of health, wellbeing and functioning outcomes. Results Three hundred sixty-four individuals completed the survey. Mean index score for the ICECAP-A was 0.761; 12 (3%) individuals reported full capability (upper anchor; score = 1), and there were no reports of zero capabilities (lower anchor; score = 0). The strongest correlations were dominated by items and dimensions on the comparator (HRQoL) instruments that are non-health aspects of quality of life, such as happiness and control over one's life (including self-care). Of 21 hypothesised constructs, 19 were confirmed in statistical tests, the exceptions being the exploratory hypotheses regarding education and age at injury. Conclusion The ICECAP-A is an empirically valid outcome measure for assessing capability wellbeing in people with spinal cord injury living in a community setting. The extent to which the ICECAP-A provides complementary information to preference-based HRQoL instruments is dependent on the comparator.