LAUSR

The Movement Disorder Society (MDS) Uni fi ed Parkinson ’ s Disease Rating Scale (UPDRS) was developed to cover the motor and nonmotor aspects of Parkinson ’ s disease (PD), and its Part III measures objectively observed motor signs with 33 items. 1 Prior item response theory (IRT) analyses of Part III con-fi rmed 2-domain tremor and nontremor constructs, each with a distinct relationship to overall PD severity. 2,3 Given that tremor and nontremor signs of PD may respond differently to medication ( on vs. off states), it is clinically and statistically important to test if the 2-domain construct is retained in both conditions. From the MDS-sponsored Scale Translation program, 4 we used full MDS-UPDRS scores from 7963 PD patients with medication state registered (6218 on and 1745 off ). We applied IRT modeling to estimate discrimination parameters using the R package mirt (R Foundation for Statistical Computing, Vienna, Austria). A higher discrimination value means that the item is more powerful for determining the individual ’ s overall parkinsonian severity, 2 and its magnitude can be judged with the following thresholds: none = 0, very low = 0.01 to 0.34, low = 0.35 to 0.64, moderate = 0.65 to 1.34, high = 1.35 to 1.69, very high ≥ 1.70. 5 We tested the internal consistency of the 2-domain overall structure in the on versus off states separately. Speci fi cally, we fi t 2 IRT models to 23 nontremor items (items 3.1 – 3.14 measuring bradykinesia, rigidity, gait, and posture, with a total score range of 0 – 92) and 10 tremor items (items 3.15a – 3.18 measuring tremor, with a