Testing the validity of the PDQ-39 in patients with MSA.

We assessed the validity of the PDQ-39, a disease-specific health-related quality of life instrument for patients with Parkinson's disease, in patients with multiple system atrophy (MSA). Two hundred and seventy-nine patients completed the PDQ-39, the EQ-5D, the Hospital Anxiety and Depression Scale, and scales of life satisfaction and disease severity. Ceiling and floor effects were noted in some dimensions, and Mobility was skewed towards the severe end of the spectrum. Apart from the dimension of Social Support, all dimensions had high internal consistency. The factor structure of the PDQ-39 in MSA was stable, and convergent and divergent validity with other measures of quality of life and mental health were good. However, many of the specific features of MSA are not reflected in the PDQ-39. Higher order factor analysis did not support the use of a single summary index. We conclude that the PDQ-39 has only limited validity in patients with MSA.