For a study, the researchers adapted a widely used instrument, the Clinical Dementia Rating (CDR) scale, which was a component of the Uniform Data Set used by all federally funded Alzheimer Disease Centers for use in adults with down syndrome (DS), and tested the instrument among 34 DS patients recruited from the community. Both modified questionnaire and interview instruments captured a range of cognitive impairments, a majority of which were found to be chronic when accounting for premorbid function. About two patients in the sample were strongly suspected to have early dementia, both of whom had elevated scores on the modified CDR instruments. Among individuals rated as having no dementia based on the dementia scale for down syndrome (DSDS), about half showed subthreshold impairments on the modified CDR instruments; there was substantial agreement between caregiver questionnaire screening and in-person interviews of caregivers and DS adults. The modified questionnaire and interview instruments captured a range of impairment in DS adults, including subthreshold symptomatology, and the instruments provided complementary information relevant to the ascertainment of dementia in DS. The decline was determined across all cognitive domains and was generally positively associated with age and negatively associated with IQ. Most importantly, adjusting instrument scores for chronic, premorbid impairment drastically shifted the distribution toward lower (no impairment) scores.