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Background: Minimal clinically important difference (MCID) is the smallest change in an outcome, which a patient identifies as meaningful. Although the 2 most frequently applied Parkinson's disease (PD) “quality of life” questionnaires (the PDQ-39 and PDQ-8) provide encouragingly similar results, their MCID thresholds appear to be vastly different. Our aim was to calculate the MCID estimates for both PDQ-39 and PDQ-8 Summary Indices (PDQ-39-SI and PDQ-8-SI) by the utilization of both anchor- and distribution-based techniques. Methods: Nine hundred eighty-five paired investigations of 365 patients were included. Three different techniques were used simultaneously to calculate the MCID values. Results: First, we replicated the previously published results demonstrating how both PDQ-39-SI and PDQ-8-SI provide similar values and respond in a similar way to changes. Subsequently, we calculated the MCID thresholds. The most optimal estimates for MCID thresholds for PDQ-39-SI were -4.72 and +4.22 for detecting minimal clinically important improvement and worsening. For PDQ-8-SI, these estimates were -5.94 and +4.91 points for detecting minimal clinically important improvement and worsening respectively. Conclusions: Our study is the first one that directly compared the MCID estimates for both PDQ-39-SI and PDQ-8-SI on a large pool of patients including all disease severity stages. These MICD estimates varied across PD severity.

Our minimal clinically important difference thresholds can be utilized in clinical practice in judging clinical relevance. © 2016 International Parkinson and Movement Disorder Society.

Sleep problems are one of the most common non-motor symptoms of Parkinson's disease (PD). The Parkinson's disease Sleep Scale 2 nd version (PDSS-2) was published in 2011 showing satisfactory clinimetric results. We performed an independent testing of the scale adding further information on its clinimetric properties.In this nationwide study 537 PD patients were enrolled. Besides PDSS-2, we assessed Patient's Global Impression-Severity (PGI) scale on sleep disturbances, Non-motor Symptoms Scale and MDS-UPDRS.Following the Classical Theory of Tests we performed descriptive data analysis, factor analysis, reliability, validity and precision measurements. Subsequently, we evaluated cut-off value for detecting clinically meaningful sleep problems based on receiver operating characteristics analysis.Based on the PGI scale, 161 patients (30.0%) did not reported any sleep problems. Factor analysis revealed almost the same factor structure described by the original PDSS-2 validation study. Cronbach's alpha was 0.863 and all item had good item-total correlation. PDSS-2 demonstrated high convergent validity with Non-Motor Symptoms Scale and Clinical Global Impression-Severity and non-motor part of MDS-UPDRS, and divergent validity with age, gender, education-level, disease-duration and Hoehn-Yahr Stages.Presence of sleep problems was identified by scores >10.5 points on PDSS-2 (sensitivity: 85.3%, specificity: 60.8%, diagnostic accuracy: 78.1%); whereas scores >19.5 points indicated marked sleep-related problems (specificity: 68.5%, sensitivity: 78.0%, diagnostic accuracy: 74.3%).Independent and cross-cultural validation of patient reported outcomes is essential to confirm or reject the findings obtained by the developers of the scale. Our results demonstrate that fundamental clinimetric properties of the PDSS-2 are satisfactory.