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The accuracy with which claims-based diagnostic codes capture Alzheimer disease and related dementias (ADRD) may vary across health care settings,1 thereby affecting the validity of research, risk adjustment, or population health management strategies predicated on these measures. Ascertainment of ADRD using diagnostic codes from hospital encounters may be biased by fragmented access to relevant clinical information, the presence of acute illnesses that alter or preclude clinical assessments,2-4 and the inclusion of problematic diagnostic codes within common claims-based definitions of ADRD. Delirium, a potential neuropsychiatric complication of hospitalization,5 may be conflated with dementia and is included in the common US Centers for Medicare & Medicaid Services Chronic Conditions Warehouse (CCW) claims-based definition of ADRD. To illustrate, we described the performance of the CCW ADRD definition in discriminating clinician-adjudicated disease status across health care settings.