Changes in Quality of Life in Parkinson's Disease: How Large Must They Be to Be Relevant?

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.