Objective: To establish whether summing scores of Movement Disorder Society– sponsored revision of the Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) Part 3 (Motor Examination) with other MDS-UPDRS Parts is clinimetrically valid.

Background: The MDS-UPDRS is composed of 4 Parts: Part 1 – Non-motor Aspects of Experiences of Daily Living, Part 2 – Motor Aspects of Experiences of Daily Living, Part 3 – Motor Examination, Part 4 – Complications of therapy. Parts 1 and 2 represent the voice of the patient and Parts 3 and 4 are administered by the clinician. The original clinimetric analyses by the MDS-UPDRS developers did not confirm the validity of summing the scores of the individual Parts. Recently, the summed score of Part 3 and other Parts has been used as primary outcome measures in various studies. Using a large sample of cross-sectional MDS-UPDRS scores, we assessed the validity of combining the Motor Examination (Part 3) with measures of non-motor (Part 1) and motor (Part 2) functional impact as well as complication of therapy (Part 4), separately or in various combinations.

Method: Using 7466 full MDS-UPDRS scores, we applied a two-step factor analysis that was used in the original validation study.  Exploratory factor analyses (EFA) were followed by confirmatory factor analyses (CFA) to determine the clinimetric validity of combining Part 3 with other Parts of the MDS-UPDRS. The criterion for success was set at a Comparative Fit Index (CFI) ≥0.9 from the CFA.

Results: The CFIs of any combination of Part 3 with other parts of the MDS-UPDRS were lower than the 0.90 criterion. CFIs ranged from a high of 0.893 for the Part 2+3 combination to a low of 0.852 for the Part 1+3+4 combination.

Conclusion: Whereas summing parts of the MDS-UPDRS may seem clinically logical, the MDS-UPDRS Parts do not have the same calibration nor the same number of items, leaving a simple summation fraught with statistical liabilities. While we continue to test alternative statistical modeling that may provide a path forward to combining the voice of the patient with clinician ratings in the MDS-UPDRS, particularly pertinent to regulatory agencies and clinical trial design, we ask our colleagues to use the MDS-UPDRS as it was envisioned and soundly tested.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/mds-updrs-part-3-cannot-be-summed-with-other-parts-of-the-mds-updrs-a-clinimetric-evaluation/