Measuring Dementia of the Alzheimer Type More Precisely



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Alzheimer?s disease (AD) progressively impairs cognitive and functional abilities. Research on pharmacological treatment of AD is shifting to earlier forms of the disease, including preclinical stages. However, assessment methods traditionally used in clinical research may be inappropriate for these populations. The Alzheimer Disease Assessment Scale-cognitive (ADAS-cog), a commonly used cognitive battery in AD research, is most sensitive in the moderate range of cognitive impairment. It focuses on immediate recall and recognition aspects of memory rather than retention and delayed recall. As clinical trials for dementia continue to focus on prodromal stages of AD, instruments need to be retooled to focus on cognitive abilities more prone to change in the earliest stages of the disease. One such domain is delayed recall, which is differentially sensitive to decline in the earliest stages of AD. A supplemental delayed recall subtest for the ADAS-cog is commonly implemented, but we do not know precisely where along the spectrum of cognitive dysfunction this subtest yields incremental information beyond what is gained from the standard ADAS-cog. An item response theory (IRT) approach can analyze this in a psychometrically rigorous way. This study?s aims are twofold: (1) to examine where along the AD spectrum the delayed recall subtest yields optimal information about cognitive dysfunction, and (2) to determine if adding delayed recall to the ADAS-cog can improve prediction of functional outcomes, specifically patients? ability to complete basic and instrumental activities of daily living.

Results revealed differential functioning of ADAS-cog subtests across the dimension of cognitive impairment. The delayed recall subtest provided optimal information and increased the ADAS-cog?s measurement precision in the relatively mild range of cognitive dysfunction. Moreover, the addition of delayed recall to the ADAS- cog, consistent with my hypothesis, increased covariation with instrumental but not basic activities of daily living. These findings provide evidence that the delayed recall subtest slightly improves the ADAS-cog?s ability to capture information about cognitive impairment in the mild range of severity and thereby improves prediction of instrumental functional deficits.