BACKGROUND: Accurate measurement of any constructs in clinical studies is of critical importance, especially if the adoption of an intervention relies on detecting a significant treatment effect where one exists. Under Neutral theory, the amount of relevant and irrelevant indicators selected to operationalise the construct contribute equally to the accuracy of the observation. The Neutral or accurate observation is achieved by observing all relevant indicators only. Generic QoL instruments such as EQ-5D are increasingly being accepted as imprecise, especially in rare diseases, based on the relevance of their indicators. QoL is a construct that embodies a patient's subjectivity, individuality, and local circumstances at measurement. SEIQoL-DW is an instrument designed to respect these characteristics of QoL through eliciting indicators or cues directly from the subject along with the proportion of the overall QoL they contribute. EQ-5D and SEIQoL can therefore be considered as being at opposing ends of accuracy in QoL measurement. XLH is a hereditary, progressive, rare disease characterised by phosphate wasting, affecting both children and adults and impacting their QoL. The purpose of this study was to observe if any change in QoL of adult XLH patients were detectable using EQ-5D, SEIQoL eliciting new cues at each visit, and SEIQoL administering baseline cues overall visits (thereby silencing its time-dependency) versus baseline over 12 months. In addition, any association between the three sets of observations was explored. RESULTS: All quality of life scores were observed to decrease from baseline by 13.36%, 7.32% and 2.7% based on SEIQoL(visit_cues), SEIQoL(baseline_cues), and EQ-5D assessments, respectively. The decrease in the quality of life scores was only statistically significant (p = 0.037) for SEIQoL(visit_cues). Beyond the baseline visit, the only highly positive and statistically significant pairwise association was between SEIQoL(visit_cues) and SEIQoL(baseline_cues) at M6 (ρ = 0.782, P value < 0.05) and M9 (ρ = 0.879, P value < 0.05). CONCLUSIONS: EQ-5D and SEIQoL(baseline_cues) failed to detect the same statistically significant decrease in QoL observed by SEIQoL(visit_cues). Both sets of SEIQoL observations were more closely associated with each other than with EQ-5D. Observing constructs such as QoL in rare diseases benefit from a Neutrality in indicator selection and respecting variation in dominance of various indicators over time.
Concordance between the schedule for the evaluation of individual quality of life-direct weighting (SEIQoL-DW) and the EuroQoL-5D (EQ-5D) measures of quality of life outcomes in adults with X-linked hypophosphatemia