The Parkinson's Disease Questionnaire (PDQ-39): development and validation of a Parkinson's disease summary index score

Objectives: to briefly outline the development and validation of the Parkinson's Disease Questionnaire (PDQ-39) and then to provide evidence for the use of the measure as either a profile of health status scores or a single index figure. Design: the PDQ-39 was administered in two surveys. a postal survey of patients registered with local branches of the Parkinson's Disease Society of Great Britain (n = 405) and a survey of patients attending neurology clinics for treatment for Parkinson's disease (n = 146). Data from the eight dimensions of the PDQ-39 were factor-analysed. This produced a single factor on the data from both surveys. Outcome measures: the eight dimensions of the PDQ-39 and the new single index score-the Parkinson's disease summary index (PDSI), together with clinical assessments (the Columbia rating scale and the Hoehn and Yahr staging score). Results: in the postal survey 227 patients returned questionnaires (58.2%). All 146 patients approached in the clinic sample agreed to take part. Higher-order principal-components factor analysis was undertaken on the eight dimensions of the PDQ-39 and produced one factor on both datasets. Consequently it was decided that the scores of the eight domains could be summed to produce a single index figure. The psychometric properties of this index were explored using reliability tests and tests of construct validity. The newly derived single index was found to be both internally reliable and valid. Discussion: data from the PDQ-39 can be presented either in profile form or as a single index figure. The profile should be of value in studies aimed at determining the impact of treatment regimes upon particular aspects of functioning and well-being in patients with Parkinson's disease, while the PDSI will provide a summary score of the impact of the illness on functioning and well-being and will be of use in the evaluation of the overall effect of different treatments. Furthermore, the PDSI reduces the number of statistical comparisons and hence the role of chance when exploring data from the PDQ-39.