Transitions in cognitive test scores over 5 and 10 years in elderly people: Evidence for a model of age-related deficit accumulation

Arnold Mitnitski¹^,2 and Kenneth Rockwood¹

¹Department of Medicine & Geriatric Medicine Research Unit, Dalhousie University & QEII Health Sciences Centre, Capital District Health Authority, Halifax, Nova Scotia, Canada

²Department of Mathematics and Statistics, Dalhousie University, Halifax, Nova Scotia, Canada

author email corresponding author email

BMC Geriatrics 2008, 8:3doi:10.1186/1471-2318-8-3


Published:	18 February 2008

Abstract

Background

On average, health worsens with age, but many people have periods of improvement. A stochastic model provides an excellent description of how such changes occur. Given that cognition also changes with age, we wondered whether the same model might also describe the accumulation of errors in cognitive test scores in community-dwelling older adults.

Methods

In this prospective cohort study, 8954 older people (aged 65+ at baseline) from the Canadian Study of Health and Aging were followed for 10 years. Cognitive status was defined by the number of errors on the 100-point Modified Min-Mental State Examination. The error count was chosen to parallel the deficit count in the general model of aging, which is based on deficit accumulation. As with the deficit count, a Markov chain transition model was employed, with 4 parameters.

Results

On average, the chance of making errors increased linearly with the number of errors present at each time interval. Changes in cognitive states were described with high accuracy (R²= 0.96) by a modified Poisson distribution, using four parameters: the background chance of accumulating additional errors, the chance of incurring more or fewer errors, given the existing number, and the corresponding background and incremental chances of dying.

Conclusion

The change in the number of errors in a cognitive test corresponded to a general model that also summarizes age-related changes in deficits. The model accounts for both improvement and deterioration and appears to represent a clinically relevant means of quantifying how various aspects of health status change with age.