A new study suggests errors on memory and thinking tests could be used to predict Alzheimer's as far as 18 years before the earliest symptoms are detectable.

The findings showed participants who scored lower on these cognitive tests were about 10 times more likely to develop the disease than those who scored higher, the American Academy of Neurology (AAN) reported.

"The changes in thinking and memory that precede obvious symptoms of Alzheimer's disease begin decades before," said study author Kumar B. Rajan, with Rush University Medical Center in Chicago. "While we cannot currently detect such changes in individuals at risk, we were able to observe them among a group of individuals who eventually developed dementia due to Alzheimer's."

In the study, 2,125 European-American and African-American people from Chicago who had an average age of 73 and did not have Alzheimer's disease were asked to take memory and thinking tests every three years for a period of 18 years. During the study period, 23 percent of the African-American participants 17 percent of European-Americans developed Alzheimer's disease. The findings showed one unit lower performance on the cognitive test was linked to an 85 percent greater risk of future dementia.

"While that risk is lower than the same one unit lower performance when measured in the year before dementia assessment, the observation that lower test scores 13 to 18 years later indicates how subtle declines in cognitive function affect future risk," Rajan said. "A general current concept is that in development of Alzheimer's disease, certain physical and biologic changes precede memory and thinking impairment. If this is so, then these underlying processes may have a very long duration. Efforts to successfully prevent the disease may well require a better understanding of these processes near middle age."

The findings were published in a recent edition of the journal Neurology.