P2-281: Differential item functioning and the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog) among people with Alzheimer's disease

The ADAS-Cog is a well-recognized measure of cognitive assessment in patients with Alzheimer's Disease (AD). It has been suggested some items and subscales of the ADAS-Cog may be better measures of cognitive functioning for different stages of AD. Despite the popularity of the ADAS-Cog, few studies have examined its item properties among patients with Mild Cognitive Impairment (MCI) and moderate impairment, and further validation of the ADAS-Cog is needed to substantiate its use at different stages of AD. This study examined whether there are differences in response to a particular item as a function of respondent characteristics (Apolipoprotein E and Impairment level) in the ADAS-Cog. jMetrik was used to analyse the ADAS-Cog. AD data was obtained from the Critical Path Institute Online Data Repository (CODR). Separate Rasch analyses were conducted comparing ApoE carriers (n = 505) vs non-carriers (n = 507), and MCI (MMSE = 21 to 26; n = 1362) and Moderate impairment (MMSE 14 to 20; n = 1211) to examine summary and individual model fit statistics, person separation index (PSI), response format, local dependency, targeting, item bias (or differential item functioning -DIF), and dimensionality Based on the results of Rasch analyses different approaches can be taken to account measurement bias in properties of the ADAS-Cog post-hoc. Lower item calibration (Delta) reflects items with bias, indicating whether subgroups respond to items differently. The average age of 74.08 years (SD = 8.15) with 55.7% male patients. Commands (Delta = 0.23), Constant Praxis (0.26), Ideational Praxis (0.32), and Naming Objects (0.26) shows 'moderate' DIF, favoring the Moderately impaired group, indicating that this item functions better in this group. Orientation (Delta = 1.34) shows 'Large' DIF, favoring the Moderately impaired group, indicating that this item functions better in this group. Word Recognition (-1.38) and Word Recall (-1.20) shows 'Large' DIF, favoring the MCI group, indicating that this item functions better in this group. The choices made during analysis will substantially affect the results, and we have described and illustrated that the subgroups may have different impact on different items affecting outcome.