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.
Hagell
Peter
P
Department of Health Sciences, Lund University, PO Box 157, SE-221 00 Lund, Sweden. Peter.Hagell@med.lu.se
Nygren
Carita
C
D000203:Activities of Daily Living / Q000523:psychology*; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000978:Antiparkinson Agents / Q000627:therapeutic use; D003072:Cognition Disorders / Q000175:diagnosis* / Q000188:drug therapy / Q000523:psychology; D004185:Disability Evaluation; D019317:Evidence-Based Medicine; D005260:Female; D015577:Geriatric Assessment / Q000706:statistics & numerical data*; D006801:Humans; D007980:Levodopa / Q000627:therapeutic use; D008297:Male; D008875:Middle Aged; D051346:Mobility Limitation; D010300:Parkinson Disease / Q000175:diagnosis* / Q000188:drug therapy / Q000523:psychology; D011594:Psychometrics / Q000706:statistics & numerical data; D011788:Quality of Life / Q000523:psychology*; D015203:Reproducibility of Results; D012803:Sick Role; D011795:Surveys and Questionnaires; D013548:Sweden
D000978:Antiparkinson Agents; D007980:Levodopa