Mistakes made on memory and thinking tests may be early warning signs of Alzheimer's disease, researchers report.

Low scores on such tests could serve as a "red flag" for the progressive brain disease up to 18 years before it can be diagnosed, the study authors added.

"The changes in thinking and memory that precede obvious symptoms of Alzheimer's disease begin decades before," study author Kumar Rajan, of Rush University Medical Center in Chicago, said in a news release from the American Academy of Neurology.

"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," Rajan said.

The study involved more than 2,000 black and white people from Chicago. The participants were an average age of 73. None were diagnosed with Alzheimer's at the start of the study, and each completed tests of memory and thinking skills every three years over nearly two decades.

During the study period, 23 percent of the black people and 17 percent of the white people developed Alzheimer's. Those who had the lowest test scores were at greater risk for the disease.

After the first year, those with lower test scores were about 10 times more likely to develop Alzheimer's than those with the best scores. These odds increased as the scores dropped below average, the study published online June 24 in the journal Neurology found.

"A general current concept is that in development of Alzheimer's disease, certain physical and biologic changes precede memory and thinking impairment," Rajan said. "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."