Objective: Test if DIF due to gender or age is present in MDS-UPDRS items.

Background: Testing a rating scale for DIF is a core step in comprehensive validation methodology. DIF occurs for the MDS-UPDRS when the probability of item scores differs among people with similar levels of parkinsonism but belong to different groups on a secondary trait (gender or age). If DIF is present, interpretation of an item score needs to include consideration of the secondary trait as well as parkinsonism severity. There are 2 types of DIF: uniform (U-DIF), where the influence on item scores by the secondary trait is constant over all levels of parkinsonism; and non-uniform (NU-DIF), where the influence on item scores by the secondary trait varies across levels of parkinsonism.

Methods: Using the cross-sectional MDS-UPDRS translation database (N=5,476), we first confirmed unidimensionality of each MDS-UPDRS Part using CFA techniques. We then tested the impact of gender and age (28-51, 52-75, 76-97 yrs) on U-DIF and NU-DIF for each Part. We required that two independent methods, MIMIC and lordif, both identified item-specific DIF to qualify for consideration. Because very few of those patients studied had scores of 4, we collapsed scores of 3 and 4 into one category to allow the methods to converge mathematically. The DIF impact was determined by McFadden pseudo R² cut-offs (large, moderate, negligible) and considered items pertinent if they exceeded the criteria beyond negligible R² > 0.035.

Results: For most MDS-UPDRS items, there was no gender- or age-based NU-DIF or U-DIF. For gender, if DIF occurred, impact was always negligible. For age, no item met the criteria for pertinent impact of NU-DIF. Two items from Part 2 (Motor Experiences of Daily Living) showed U-DIF of moderate impact: 2.11 Getting out of bed (R²=0.060) and 2.12 Walking and balance (R²=0.053).

Conclusions: Gender has no pertinent DIF impact for the MDS-UPDRS, allowing the scale to be utilized as a core outcome measure across populations with varying gender distributions. For age, items 2.11 and 2.12 demonstrated pertinent U-DIF. As subject age increases, higher scores on these two activities occur at all levels of overall Part 2 severity. Interpretation of these items needs to include consideration of patient age as well as parkinsonism in evaluating MDS-UPDRS Motor Experiences of Daily Living (Part 2).

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MDS Abstracts - https://www.mdsabstracts.org/abstract/gender-and-age-based-differential-item-functioning-dif-analysis-of-mds-updrs/