Mapping the eight-item Parkinson's Disease Questionnaire (PDQ-8) to the EQ-5D utility index

To develop a function for mapping the eight-item Parkinson's Disease Questionnaire (PDQ-8) to the EuroQol Group's EQ-5D utility index.

Data from two surveys of 324 patients with Parkinson's disease was divided into two groups. One was used to estimate the mapping functions by regression methods and the other was used to validate the mapping functions.

A regression model with a non-linear trend explained 55% of the variation in EQ-5D utility values and had a mean absolute deviation (MAD) of 0.083. A regression model assuming a linear trend explained 52% of the variation and had an MAD of 0.085. In the validation sample, predicted values based on the aforementioned models respectively explained 42 and 44% of the variation in the observed EQ-5D utility values and both had MADs of about 0.1. The confidence intervals of the mean difference between these predicted values and the observed values totally fell within the pre-defined equivalence margin of 0.03 points. These predicted values were also similar to the observed EQ-5D utility values in terms of their association with clinical variables.

At the group level, but not at the individual level, the mapping functions can accurately map the PDQ-8 outcomes to the EQ-5D utility index.