Validity and reliability of the PDQ-39 and the PDQ-8 in English-speaking Parkinson's disease patients in Singapore

Abstract

The purpose of the study was to assess the validity and reliability of the Parkinson's disease questionnaire (PDQ-39, UK English version) as well as its briefer version (the PDQ-8) among Asian patients with Parkinson's disease (PD) in Singapore. Eighty-eight patients recruited from movement disorder clinics or patient support groups completed the PDQ-39 and EQ-5D, a generic health related quality of life questionnaire previously validated for use in Singapore. PDQ-39 items showed good convergent and discriminant validity. Construct validation against the EQ-5D showed strong correlation between these scales as hypothesised (Spearman's ρ: 0.53–0.71, P<0.001 for all). Factor analysis yielded a component on which all eight PDQ-39 dimensions were substantially loaded (loading range: 0.50–0.79). The PDQ-39 also demonstrated acceptable internal consistency (Cronbach's α: 0.59–0.94) and test–retest reliability (intra-class correlation coefficient:0.67–0.87, n=68). Six of eight PDQ-8 items showed the strongest correlation with their PDQ-39 dimensions as hypothesised. PDQ-39 and PDQ-8 summary indexes were highly correlated (Pearson's r=0.96, ICC=0.95), and both were strongly correlated with the EQ-5D utility index and EQ-5D visual analogue scale (P<0.001). We conclude that the PDQ-39 and PDQ-8 are valid and reliable disease-specific HRQoL instruments for PD in Singapore.