Measuring the impact of drug-induced dyskinesias in Parkinson’s disease: the PDYS-26 revisited

BACKGROUND: Drug-induced dyskinesias (DID) are a common long-term complication of dopaminergic drug therapy for Parkinson’s disease (PD). The 26-item PD dyskinesia scale (PDYS-26) is a patient-reported rating scale intended to quantify the everyday impact of DID. However, its measurement properties have not been scrutinized since its development some 10 years ago.

AIM: To examine the measurement properties of the PDYS-26 using Rasch Measurement Theory (RMT).

DESIGN: The PDYS-26 was administered to people with PD and varying degrees of DID recruited from six Swedish (n=172) and three British (n=150) outpatient movement disorder clinics. RMT analyses were conducted using the RUMM2030 software.

RESULTS: RMT model fit was generally good with only three items exhibiting relatively minor misfit. Response categories worked as intended and targeting was acceptable and reliability was 0.96. There was no differential item functioning (DIF) by age, PD duration or time, but three items exhibited DIF by country and one by sex, neither of which appear to notably bias person measurement. Item hierarchy review suggested a variable of dubious clinical/theoretical coherence. Therefore, a recently proposed three-dimensional reduced (21-item) PDYS version was explored. Results were similar to those from the original scale but with improved conceptual coherence, albeit with close to a 2- to 3-fold decrease in measurement precision. 

SUMMARY AND IMPLICATIONS: The PDYS-26 appears useful for measuring the impact of DID, and yields sufficiently invariant measurement across core patient subgroups. Scale reconceptualization improves the meaning of scores, but at the cost of measurement uncertainty.