A new study suggests that errors on memory and thinking tests may signal Alzheimer's up to 18 years before the disease can be diagnosed. The research is published in the June 24, 2015, online issue of Neurology®, the medical journal of the American Academy of Neurology.

"The changes in thinking and memory that precede obvious symptoms of Alzheimer's disease begin decades before," said study author Kumar B. Rajan, PhD, 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."

For the study, 2,125 European-American and African-American people from Chicago with an average age of 73 without Alzheimer's disease were given tests of memory and thinking skills every three years for 18 years.

Twenty-three percent of African-Americans and 17 percent of European-Americans developed Alzheimer's disease during the study. Those who scored lower overall on the memory and thinking tests had an increased risk of developing the disease. During the first year of the study, people with lower test scores were about 10 times more likely to be diagnosed with Alzheimer's disease than people with higher scores, with the odds increasing by 10 for every standard deviation that the score was lower than the average.

Based on tests completed 13 to 18 years before the final assessments took place, one unit lower in performance of the standardized cognitive test score was associated with an 85 percent greater risk (relative risk of 1.85) 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," said Rajan.

"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," Rajan said.