Initial symptoms of Alzheimer’s disease can appear up to 18 years before it is actually diagnosed, according to a new study published in Neurology journal. And, low scores on memory and thinking tests can predict the risk of the neurodegenerative disease much before the diagnosis, the study found.

“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,” study author Kumar B. Rajan, assistant professor with Rush University Medical Center in Chicago said, in a press release.

In the 18-year-long study, researchers examined 2,125 European-Americans and African-Americans by giving them memory and thinking ability tests. The average age of the participants was 73 and they were required to take the tests every three years.

The researchers found that 23 percent of African-Americans and 17 percent of European-Americans developed Alzheimer’s during the study time frame. The team noted that participants scoring less in the memory and thinking tests in the first year of the study had 10 times increased risk of developing Alzheimer’s compared to those who scored well in the tests.

According to the researchers, the tests completed between 13 and 18 years of the final evaluation showed the greater risk of the disease. The team observed that one fewer unit in test performance was linked to an 85 percent increased risk of Alzheimer’s, the most common form of dementia.

“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, in the release.

Statistics from the Alzheimer’s Foundation of America estimate that about 5.1 million Americans likely have the disease, which is on the rise as the country's population ages. According to the estimates, about half a million Americans below the age of 65 have some form of dementia, including Alzheimer's disease.