Misclassification is likely in the assessment of mild cognitive impairment

We estimated the relative frequency of isolated memory impairment versus isolated and comorbid impairment in executive control function (ECF). One hundred and ninety-three noninstitutionalized residents of a single Comprehensive Care Retirement Community (mean age 79.2 years) were investigated. The subjects were tested with multiple measures of memory and ECF. Test scores were standardized to minimize scaling effects. 'Impairment' was defined as performance < or =1.5 standard deviations below the mean for the entire sample (i.e., a z score < or =-1.5). Disability was estimated as the sum of self-reported activities of daily living and instrumental activities of daily living. The cognitive test performance was significantly associated with functional impairment, independently of age. ECF and memory measures were significantly intercorrelated. Both were significantly and independently associated with disability ratings. 6-10% of the subjects had memory impairment; 25-35% of the memory-impaired subjects had comorbid ECF impairments. An additional 4-7% of the subjects had isolated ECF impairment. A significant fraction of the cases otherwise meeting the criteria for 'mild cognitive impairment' may have comorbid ECF impairment. This raises the issue of whether they might be more properly classified as 'demented'. In addition, isolated ECF impairment may affect almost as many persons as isolated memory impairment. Isolated ECF impairment is not consistent with the natural history of preclinical Alzheimer's disease, suggests other conditions, and can be disabling, independently of age and/or memory loss.