Errors in simple memory and cognitive tests can predict a person's risk of developing Alzheimer's disease up to 18 years before it can be properly diagnosed, researchers in the US report.

In a study of over 2,000 participants, a team from Rush University Medical Centre in Chicago found that those who got the lowest scores in the tests over an 18-year period were almost 10 times more likely to be diagnosed with Alzheimer's disease than people with higher scores. They say that by focussing on the very subtle changes in brain function that occur several decades before Alzheimer's symptoms show up, we could figure out how to better treat, or even prevent, this devastating disease.

The team worked with 2,125 European-American and African-American volunteers living in Chicago with an average age of 73. None of them had been diagnosed with Alzheimer's disease when they were selected for the study. The participants were given exercises that tested both their memory and cognitive skills every three years over an 18-year period.

During the course of the study, 23 percent of the African-American participants and 17 percent of European-American participants - so 21 percent of the participants overall - ended up developing Alzheimer's disease. When the researchers looked back at their test scores, they found that those on the high and low end of the spectrum could be used to predict a participant's risk of developing Alzheimer's in the future. 

According to the paper, which has been published in the journal Neurology, during the first year of the study - 18 years before any of them were diagnosed - those who scored lower overall in the two types of tests were 9.84 times more likely to be diagnosed with Alzheimer's disease than those with higher scores. These odds increased by 10 for every standard deviation that the score was lower than the average throughout the years that followed.

"The most telling period of the study was for memory and thinking tests completed between 13 and 18 years before the research ended," David DiSalvo reports for Forbes Magazine. "Every single unit of lower performance on the test (a unit being a standard drop below the average score) was associated with an 85 percent greater risk of developing the disease."

Right now, it's not clear why there appears to be a correlation between low test scores and the onset of Alzheimer's disease. The researchers say the changes are so subtle, they would not have been able to detect them by taking brain scans of individual participants and tracking functional and structural changes over the next few decades of their life. The most effective way of figuring out what these changes are, or at least coming up with ways of detecting them, is to continue working with large groups of people and figure out what distinguishes those who do end up being diagnosed with the disease from those who don't, one of the researchers, Kumar B. Rajan, said in a press release. Then maybe we can come up with a standardised test that can identify at-risk groups for whatever preventative treatments might be developed in the future.

"A general current concept is that in development of Alzheimer's disease, certain physical and biological changes precede memory and thinking impairment. If this is so, then these underlying processes may have a very long duration," says Rajan. "Efforts to successfully prevent the disease may well require a better understanding of these processes near middle age."