Michael Ellenbogen knows all too well what it's like to live with Alzheimer's.

Doctors gave him that dreadful diagnosis more than a decade ago. In the years that followed, he learned to cope with his progressive memory loss, and became a fierce advocate on behalf of the 5.5 million Americans who have Alzheimer's.

Now he's adjusting to a very different reality.

The doctors were wrong; it seems he doesn't have Alzheimer's after all.

Michael Ellenbogen says he will continue his advocacy work on dementia issues, after a new form of testing found he most likely does not have Alzheimers.

Ellenbogen shared that stunning development in an essay he penned for the Washington Post last October, ruing the uncertainty and ambiguity that often come with a diagnosis of dementia.

The idea of being told you have Alzheimer's and then finding out you don't sounds like a miraculous, hope-inspiring reprieve. Yet that's not how Ellenbogen sees it.

"I don't know why anyone would think that, because it's still a progressive dementia that I have," says the 59-year-old Ellenbogen, of Bucks County, Pa. "I wouldn't say what I wrote (in the Post) was a message of hope. I would say it's more about the importance of taking advantage of all the tools available today, and that you really can't trust what you're being told because nobody knows for sure what you have."

Ironically, he says, it was a "relief'' to get his diagnosis of Alzheimer's back in 2008, because at least he finally knew what was wrong with him -- or so he thought.

Now, it's a relief to not have Alzheimer's. But the long, strange journey of the past 10 years has plunged him back into the murkiness of no clear diagnosis. He may have semantic dementia. He may have a mixed dementia. Or perhaps some type of dementia that doesn't even have a name yet.

"I will have to wait for science to catch up before I know for sure," he writes in The Post. "Who knows, I may even be part of the next new term. I just hope it means that what I have will not be progressive and lead to death."

Ellenbogen has carried that ambiguity with him for a long time -- all the way back to the age of 39, when he was first diagnosed with mild cognitive impairment.

He was an unlikely candidate for it, exhibiting none of the usual risk factors. He was well-educated, doing mentally challenging work as an entrepreneur and network operations manager for an East Coast technology company. And he got plenty of exercise. "I used to walk up 11 flights of stairs at work all the time," he says. "I was always healthy, and thin."

But his memory was slipping, and he was having other cognitive problems. One doctor eventually diagnosed Alzheimer's; another said it was semantic dementia. After an additional year of testing, the diagnosis became firm: Alzheimer's.

"Overwhelmed, I decided to help the search for a cure by advocating for Alzheimer's and dementia," he writes.

Ellenbogen got involved in clinical trials and advocacy. He wrote a book, "From the Corner Office to Alzheimer's." He spoke nationally and internationally. His Facebook page, The Michael Ellenbogen Movement, bills itself as "a place for serious Alzheimer's discussion."

But even as Ellenbogen threw himself deeply in the cause of Alzheimer's, he began to question his diagnosis. He says he had symptoms -- impatience, over-reaction to minor concerns -- that aren't typically associated with Alzheimer's. And although he took part in numerous clinical trials aimed at Alzheimer's, none of them benefited him.

"I was just very skeptical about it, because of what I'd learned," he says. "I kind of thought all along I might have some other symptoms, and it led me to think I had multiple types of dementia, not just one type. There's just no way a doctor can tell you what you have. They may know what you primarily have possibly, but even then they might be wrong."

In August of last year, he finally got an answer, thanks to a fairly recent breakthrough in the ability to detect Alzheimer's early in the progression of the disease.

It used to be that doctors might strongly suspect a patient had Alzheimer's, but there was no way to confirm that until after the patient died and an autopsy was performed.

However, that has changed in a big way. A new test called Amyvid, developed by Eli Lilly, uses a PET scan to detect the level of beta-amyloid density in the brain. These amyloid "plaques" are a hallmark of Alzheimer's. If they don't show up in the PET scan, it's a near certainty that the patient doesn't have Alzheimer's.

Once it became possible to find out, Ellenbogen wanted to know. So he volunteered for a study specifically designed to give people this test.

"Every clinical trial I was in before that, I went in because I knew somebody had to do it," he says. "I took part in those because they needed people. For this one, I was kind of selfish because I wanted the results for myself."

He had to wait two years for the test, and even after he took it, he waited another month to get the results. He says it was nerve-wracking to live in such suspense. Finally, he got the results. The test came back negative.

"Was I ever shocked," he writes. "I also felt like a fraud because, for years, I had been in the public eye telling everyone I had Alzheimer's."

Now that Alzheimer's has essentially been ruled out, Ellenbogen has a new diagnosis. He says two neurologists now put him in a category called suspected non-Alzheimer's pathophysiology, also called SNAP. He says about one-fourth of adults over the age of 65 are estimated to have SNAP, and people like him, who have both mild cognitive impairment and SNAP, are at higher risk of continued cognitive decline and dementia. Nor has the possibility of semantic dementia been ruled out.

So the risk remains real -- and he continues his advocacy work.

"I haven't changed my focus," he says. "I still focus on almost everything related to the dementia cause. I will say I'm probably a little more honed in to wanting to push the issues of the importance of amyloid PET scans and why they're so important right now."

While this newfound ability to detect Alzheimer's has the potential to profoundly change the way doctors approach treatment of the disease, so far it hasn't.

That's because these PET scans are not yet widely available. Ellenbogen says the scan can cost $10,000 and in most cases isn't covered by Medicare or private insurance. So it's rare for people to have access to it.

The unwillingness of insurers to cover these PET scans is based on a 2013 decision by a Medicare panel, which said there's not enough evidence that the test is "reasonable and necessary for the diagnosis or treatment" of dementia.

But Alzheimer's advocates are working to change that conclusion and get the tests covered. Ellenbogen says the value of doing so is obvious.

"I don't know how the insurance companies determine these things, but I think as long as you set some criteria, it may be worthwhile doing," he says.

Ellenbogen says while a PET scan is expensive, he has been through numerous rounds of neuropsychological testing, and those were expensive, too. "That cost a lot of money," he says, "so maybe instead of one of those, you go to get a PET scan."

He's more chagrined about the resources wasted by putting him through studies designed to test potential Alzheimer's drugs, when he didn't have Alzheimer's.

"When you take into consideration how much money was spent on me to be in all these clinical trials, they wasted a lot of money," he says. "They wasted not only the money, but I contributed to the bottom line in the failing of those tests."

This concern is one shared by Alzheimer's researchers. The past decade has brought a long string of disappointments, as one trial after another ended in failure. Experts say part of the problem was a flawed approach to recruiting people to take experimental drugs. Participants weren't always screened sufficiently to make sure they had Alzheimer's.

Another concern is that these experimental drugs, though promising, were tested later in the disease process, when too much damage may already have been done to the brain. Researchers are optimistic that with the PET scan, and other new diagnostic tools for detecting Alzheimer's, it's now possible to make sure people being given test drugs have Alzheimer's, and that they are receiving these drugs early enough to potentially be helped by them.

As a result, PET scans are becoming more available to people entering clinical trials.

But the bigger goal is to make them more broadly available to anyone who is suspected of having Alzheimer's, so the disease can be either confirmed or ruled out, and the patient can receive whatever treatment is most appropriate.

Right now, more than 18,000 Medicare recipients over the age of 65 who have been diagnosed with mild cognitive impairment or dementia are in the process of being tested for Alzheimer's. So far, nearly 4,000 have been tested and only about half of those with mild cognitive impairment had the plaques associated with Alzheimer's. In the majority of cases, their doctor ended up changing their treatment based on what the PET scan found.

Dr. Gil Rabinovici, of the University of California San Francisco, says patients are getting better care based on this clearer understanding of whether they have Alzheimer's.

"I can tell you as someone who cares for patients, they want to know what's going on in their brain," he said at a press briefing last year. "They want to understand what the cause of their cognitive impairment is. The value of receiving that diagnosis, even if it's bad news, is that at least it leads to a clear plan in terms of what drugs to use, or possibly participating in clinical trials, and it offers some certainty."

The study is being run by the Alzheimer's Association, and officials hope the results will ultimately persuade Medicare and insurance companies to cover the cost of a PET scan.

Ellenbogen supports this effort, saying it has the potential to dramatically improve care for Alzheimer's patients in the coming years.

"I would think it definitely will, especially as they come up with drugs that can stop it from happening, just like you take drugs for cholesterol," he says. "They are going to figure out ways to manage it before it progresses. Some of those drugs that failed for people already living with disease, they are trying it with people at much earlier stages."

Having lived a decade under the false impression that he had Alzheimer's, Ellenbogen says the PET scan changed his life, and he's determined to see it become more available to others.

"The people who don't feel there's a value are the people who don't have the damned disease," he says. "I think I proved that more than ever."

Tony Dearing may be reached at tdearing@njadvancemedia.com. Follow him on Twitter @TonyDearing. Find NJ.com on Facebook.