About 10 percent of Americans aged 71 and up, or 2.4 million people, have Alzheimer's disease and 1 million more have some other form of dementia, researchers said on Tuesday, offering figures lower than some widely cited estimates.
The National Institutes of Health, which funded the study, called it the first to gauge U.S. rates of Alzheimer's and other dementia using a nationally representative sample of elderly people.
"Most studies have been limited to a small region of the country or a few cities within the country, and then have used the findings from local regions to project the estimates of dementia," Brenda Plassman of Duke University Medical Center, one of the researchers, said in a telephone interview.
"Our study has examined people in all regions of the U.S. and then estimated the prevalence," Plassman added.
The researchers based their findings on evaluations of 856 people ages 71 and older from 42 states, all of whom were interviewed and evaluated at their homes from 2001 to 2003.
Alzheimer's is difficult to diagnose, with a conclusive diagnosis only possible with a brain biopsy. But there are tests that doctors can perform to give them a good idea.
The researchers assessed each person's cognitive status and symptoms, medications, medical history and family history of memory problems. Brain imaging was also examined. Experts then judged if a person had dementia and, if so, what type.
Alzheimer's disease accounted for about 70 percent of dementia cases among people 71 and older, with vascular dementia the next most common form, the researchers said.
The researchers estimated that 13.9 percent of Americans age 71 and older have some type of dementia, including 9.7 percent suffering from Alzheimer's. That translates to 3.4 million Americans — one in seven people age 71 and older — with dementia, including 2.4 million with Alzheimer's.
The researchers said the rate of Alzheimer's rises with age: 2.3 percent of those aged 71 to 79; 18.1 percent of those ages 80 to 89; and 29.7 percent of those ages 90 and up.
The study was published in the journal Neuroepidemiology.
A leading U.S. advocacy group, the Alzheimer's Association, in March reported that there were more than 5 million people in the United States living with the disease. The group said the number included 4.9 million people over the age of 65 and between 200,000 and 500,000 people under age 65 with early onset Alzheimer's disease and other types of dementia.
Other estimates also have been higher than in the new study.
Richard Suzman, director of the Behavioral and Social Research Program at the NIH's National Institute on Aging, said the new estimates are the best to date.
"It's somewhat lower, yes," Suzman said in a telephone interview. "There are different methodologies around so there is some level of uncertainty."
Suzman said regardless of the differences, the number of elderly Americans with Alzheimer's is large. "But that's not the number to keep your eye on. The real number is what it's going to be in 30 or 40 years because the population of people age 85 and over — who are at the highest risk for the disease — is going to quadruple or quintuple," he said.
Bill Thies, the Alzheimer's Association vice president for medical and scientific relations, played down the differences between his group's earlier estimates and the new ones, while saying the problem has already reached crisis dimensions.
"It's in the right range for all the studies," Thies said of the new data. "Do I think any one of the studies has exactly the right number? The answer is no. None of them do."
