In the United States, it’s estimated that about 7 million people are living with Alzheimer’s disease and related dementias. But the number of people with a formal diagnosis is far less than that. Now, a new study suggests the likelihood of getting a formal diagnosis may depend on where a person lives.

Researchers at the University of Michigan and Dartmouth College found that diagnosis rates vastly differ across the country and those different rates could not simply be explained by dementia risk factors, like if an area has more cases of hypertension, obesity and diabetes.

The reasons behind the disparity aren’t clear, but researchers speculate that stigma as well as access to primary care or behavioral neurological specialists may impact the odds of getting a formal diagnosis.

“We tell anecdotes about how hard it is to get a diagnosis and maybe it is harder in some places. It’s not just your imagination. It actually is different from place to place,” said Julie Bynum, the study’s lead author and a geriatrician at the University of Michigan Medical School.

Those differences may have potential consequences. That’s because a formal diagnosis of Alzheimer’s opens up access to treatments that may slow down the brain changes associated with the disease. Without that formal diagnosis, patients also would not be eligible for clinical trials or insurance coverage for certain medications. Even in cases of dementia where treatment is not an option, a diagnosis can also help in the planning for a patient’s care.

The findings, published last week in the journal Alzheimer’s & Dementia, emerged from two main questions: What percent of older adults are being diagnosed with dementia across communities in the U.S.? And is the percent we see different from what we would expect?

To answer these questions, researchers used Medicare and demographics data to create two maps. The first displayed the percentage of people receiving a formal diagnosis in each hospital referral region (HRR), which divides the country into 306 areas based on where people are likely to seek treatment. The second estimated what the percentage should be in each HRR based on health risk factors and race.

What they discovered was that the two maps were vastly different, with parts of the Great Plains and Southwest seeing less diagnosis than expected. For example, a person in Wichita Falls, Texas, may have twice the likelihood of getting a diagnosis than a person living in Minot, N.D.

“Even within a group of people who are all 80, depending on where you live, you might be twice as likely to actually get a diagnosis,” Bynum said.

It’s difficult to say for certain if an area is under-diagnosing, because researchers compared each HRR to the national diagnosis average instead of the actual number of cases in each community, she added.

But the findings shed new light on why dementia diagnosis is more prevalent in some areas than others — and that it does not simply have to do with an individual’s risk factors alone, but also access to health care resources and education on the disease.

Erin Abner, an epidemiologist at the University of Kentucky who was not involved in the study, said the results were not surprising and that there are many barriers to diagnosis.

“Where we live is a powerful influence on our brain health,” she said. “It is very difficult for adults in many parts of the country to access behavioral neurological specialist care — in many cases waiting lists to be seen are months or even years long.”

For some, language and cultural differences can also impact access to care.

Diagnosing Alzheimer’s can be a long process that includes cognitive and neuropsychological assessments, as well as tests showing the presence of amyloid plaques in the brain. Bynum hopes the findings will help draw attention to the role that health care systems have on diagnosis rates and finding people who may be living with dementia under the radar.

“This other component of what the health care system and our public health system might do in informing and educating populations, that’s also relevant and important,” Bynum said. “And in some ways, we can fix that.”