The 39 item Parkinson's disease questionnaire (PDQ-39) revisited: implications for evidence based medicine

Background

The 39 item Parkinson''s disease questionnaire (PDQ‐39) is the most widely used patient reported rating scale in Parkinson''s disease. However, several fundamental measurement assumptions necessary for confident use and interpretation of the eight PDQ‐39 scales have not been fully addressed.

Methods

Postal survey PDQ‐39 data from 202 people with Parkinson''s disease (54% men; mean age 70 years) were analysed regarding psychometric properties using traditional and Rasch measurement methods.

Results

Data quality was good (mean missing item responses, 2%) and there was general support for the legitimacy of summing items within scales without weighting or standardisation. Score reliabilities were adequate (Cronbach''s alpha 0.72–0.95; test–retest 0.76–0.93). The validity of the current grouping of items into scales was not supported by scaling success rates (mean 56.2%), or factor and Rasch analyses. All scales represented more health problems than that experienced by the sample (mean floor effect 15%) and showed compromised score precision towards the less severe end.

Conclusions

Our results provide general support for the acceptability and reliability of the PDQ‐39. However, they also demonstrate limitations that have implications for the use of the PDQ‐39 in clinical research. The grouping of items into scales appears overly complex and the meaning of scale scores is unclear, which hampers their interpretation. Suboptimal targeting limits measurement precision and, therefore, probably also responsiveness. These observations have implications for the role of the PDQ‐39 in clinical trials and evidence based medicine. PDQ‐39 derived endpoints should be interpreted and selected cautiously, particularly regarding small but clinically important effects among people with less severe problems.The past decade has seen two major developments in clinical Parkinson''s disease (PD) research: an increasing focus on evidence based medicine and a growing emphasis on the importance of patient reported outcomes.^1,2 It is therefore reasonable to expect the effectiveness of therapy to increasingly be judged on the basis of patient completed rating scales. A prerequisite for valid interpretation of clinical findings, and hence evidence based medicine, is that rating scales can be interpreted with confidence.^3,4,5,6 The need for high quality patient reported rating scales in PD and the fundamental role of evidence based measurement in clinical research is thus apparent.The 39 item PD questionnaire (PDQ‐39)⁷ is the most widely used disease specific patient completed rating scale in PD.⁸ However, several important measurement properties of the PDQ‐39 have not been fully addressed. For example, basic requirements (scaling assumptions) that determine the legitimacy of summing PDQ‐39 item scores without weighting or standardisation have not been examined, and studies addressing the validity of grouping items into its eight scales (dimensionality) have shown inconclusive or discouraging results.^9,10,11,12 This poses limitations on the possibility to interpret study outcomes as it may be unclear what scores represent.⁴ There have also been indications that the PDQ‐39 may not target respondents adequately, which could affect its ability to detect clinically relevant changes.¹⁰ Re‐evaluation of the PDQ‐39 therefore appears warranted to help inform its use and role in clinical trials and evidence based medicine.With this in mind, we assessed the scaling assumptions, reliability, dimensionality and targeting of the eight PDQ‐39 scales. Whereas the PDQ‐39 was developed within the traditional test theory framework, modern test theory (particularly the Rasch model) is increasingly considered advantageous in scale development and evaluation.^{3,13,14,15,16} The PDQ‐39 was therefore analysed using both traditional and Rasch measurement methods.