Transfer bias and the association of cognitive impairment with falls.

The authors hypothesized that cognitive impairment is associated with falls in older adults, but that transfer bias may obscure this association in cross-sectional community studies. The bias would arise if demented patients who fall are relatively unavailable to community surveys due to death or institutionalization. To test this hypothesis, a "dose-response" relationship between falls and cognitive impairment was tested for using data from a longitudinal cohort study of 157 patients with Alzheimer-type dementia. In a cross-sectional analysis of baseline data, when 96% of the cohort were community residents, the association between falls and cognitive impairment was insignificant (odds ratio for a 10-point change in Mini-mental State score = 1.2, 95% confidence interval 0.76-1.9). Yet cognitive impairment at baseline predicted falls during three-year follow-up (OR = 1.8, 95% CI 1.1-3.0). Both severity of dementia and falls were risk factors for death or institutionalization. Patients at highest risk for leaving the community during follow-up were those who became non-ambulatory. The authors conclude, based on longitudinal data, that there is a dose-response effect between cognitive impairment and falls. Transfer bias probably obscured the association at baseline. These data emphasize the importance of prospective studies of falls.