Two questionnaires for Parkinson's disease: Are the PDQ-39 and PDQL equivalent?

Background: Although there is a wide range of specific health-related quality of life (HRQoL) measures for Parkinson's disease (PD), their psychometric attributes have never been compared. Objective: To compare the psychometric characteristics of the Parkinson's Disease Questionnaire-39 items (PDQ-39) and Parkinson's Disease Quality of Life Questionnaire (PDQL). Methods: The PDQ-39 and PDQL were simultaneously applied to 187 PD patients. Additional assessments included Hoehn and Yahr staging, Schwab and England Scale (SES), Unified Parkinson's Disease Rating Scale, Pfeiffer's Short Portable Mental Status Questionnaire (SPMSQ) and Hospital Anxiety and Depression Scale (HADS). Criteria for acceptability, scaling assumptions, reliability, construct validity and precision were drawn up. Results: Both the PDQ-39 and PDQL proved very similar in the following aspects: acceptability (no floor or ceiling effect); scaling assumptions (79.5% and 80% respectively of item-total correlations fitted the standard); internal consistency (PDQ-39 domains, α = 0.43-0.93; PDQL domains, α = 0.70-0.88); stability (intraclass correlation coefficient = 0.76-0.90); and internal validity (PDQ-39, r = 0.09-0.70; PDQL, r = 0.52-0.71). Convergent validity between the two questionnaires was moderate to high (r = -0.91 for summary indices). Precision was higher for the PDQ-39 Summary Index, and predictors were the same for both measures (mood disorders and disability). Conclusions: Although the psychometric properties tested are quite similar in both measures, mainly insofar as summary indices are concerned, differences in some attributes and structural content should nevertheless be considered when comparing both instruments.