Coordinating outcomes measurement in ataxia research: do some widely used generic rating scales tick the boxes?

The objective of this study was to examine the psychometric properties of four widely used generic health status measures in Friedreich's ataxia (FA), to determine their suitability as outcome measures. Fifty-six people with genetically confirmed FA completed the Barthel Index (BI), General Health Questionnaire (GHQ-12), EuroQol (EQ-5D), and Medical Outcomes Study 36-item Short Form Health Survey (SF-36) by means of postal survey. Six psychometric properties (data quality, scaling assumptions, acceptability, reliability, validity, and responsiveness) were examined. The response rate was 97%. In general, the psychometric properties of the four measures satisfied recommended criteria. However, closer examination highlighted limitations restricting their use for treatment trials. For example, the BI had high levels of missing data, EQ-5D had poor discriminant ability, and five SF-36 scales had high floor and/or ceiling effects. Most scale scores did not span the entire scale range, had means that differed notably from the scale mid-point, and had wide confidence intervals. Effect sizes (ES) were small for all four measures raising questions about their ability to detect clinically significant change. Results highlight the potential limitations of these four scales for evaluating health outcomes in FA and suggest the need for new disease-specific patient-based measures of its impact.