Mapping of the PDQ-39 to EQ-5D scores in patients with Parkinson's disease

The EuroQoL (EQ-5D) is ideal to compare quality of life across conditions. However, the Parkinson's Disease Questionnaire (PDQ-39) is often the only quality-of-life instrument used in Parkinson's disease research. We aimed to identify associations between PDQ-39 domains and EQ-5D domains, and compare different methods of developing a function to map the PDQ-39 to EQ-5D scores. Adults with Parkinson's disease self-completed both instruments. Ordinal regression identified associations between PDQ-39 domain scores and each EQ-5D domain. Modeling (n = 80) and validation sets (n = 16) were randomly generated. Overall performance of four methods of mapping the PDQ-39 to EQ-5D scores (using PDQ-39 domains and total score in ordinal and linear regression) was assessed with the validation set, followed by assessing the equivalence of observed and predicted EQ-5D scores on the full dataset controlling for sociodemographic factors. Different sets of PDQ-39 domains were associated with each EQ-5D domain. For example, PDQ-39 Activities of Daily Living and Social Support were associated with EQ-5D Personal Care, while PDQ-39 Emotional Well-being was associated with EQ-5D Anxiety/Depression. Over one-third (37.5 %) of predictions from ordinal regressions had an error < 0.01 % (compared to 6.3 % for linear regressions). The EQ-5D scores predicted with ordinal regression using PDQ-39 domains were similar in distribution and association with sociodemographic factors to the observed EQ-5D scores. Of the four methods tested, using PDQ-39 domains in ordinal regression was superior for mapping EQ-5D scores. The function reported here may prove particularly useful for cost-utility analyses comparing Parkinson's disease with other conditions.