"Someone who has biomarker evidence of amyloid in the brain has the disease, whether they’re symptomatic or not."

"The pathology exists for years before symptom onset. That’s the science. It’s irrefutable."

Said Dr. Clifford R. Jack Jr., an Alzheimer’s researcher at the Mayo Clinic, quoted in "Apparently Healthy, but Diagnosed With Alzheimer’s? New criteria could lead to a dementia diagnosis on the basis of a simple blood test, even in the absence of obvious symptoms" (NYT)(free access link).

But Jack and his colleagues are not saying to test everyone.

A 2015 Dutch study estimated that more than 10 percent of cognitively normal 50-year-olds would test positive, as would almost 16 percent of 60-year-olds and 23 percent of 70-year-olds. Most of those individuals would never develop dementia....

What's the point of knowing you technically have this dreaded disease if you don't have the aspect of it that is dreaded — the outward symptoms?

2 things: 1. It's easy now to test —the blood tests are "highly accurate, less invasive than spinal taps and far less expensive than brain scans" — and 2. There are new drugs that have been shown to "slow the progression of symptoms over 18 months in those with mild cognitive impairment or mild Alzheimer’s disease."

If it's easy to test and there are drugs to take, won't the tests become routine for anyone over 50 (or even younger)?

Doctors diagnose many diseases, including diabetes and cancer, with tests in asymptomatic people....

And we're already talking about old people. As Dr. Jack puts is: “Because death rates are high in older people, many will die before they develop dementia.”

And the article quotes Dr. Eric Widera, a geriatrician at the University of California, San Francisco: “These are not benign drugs.... You’ll be on these drugs for the rest of your life — like a statin, but a lot more expensive and a lot more dangerous.”

Dr. Widera further criticized the working group’s proposal for not discussing the harms of the new criteria — including needlessly terrifying people unlikely to develop dementia and potentially causing discrimination in employment and insurance.

Dr. Jack, who has no reported conflicts of interest, defended his working group. “The members are committed to accurately reflecting what the current science says,” he said. “There was no consideration of commercial gain. Everyone was focused on what’s best for patients.”... 

Often, redefining diseases or revising guidelines means reducing thresholds and broadening classifications, sometimes called “diagnosis creep.” The thresholds for high blood pressure and high cholesterol are lower now than in previous years, for example. New precursor conditions like prediabetes also expand the number of people defined as having a disease....

 Dr. Jack says, "I personally would not elect to know if I had plaques in my brain" — I would (wouldn't you?) — and "he would not prescribe amyloid drugs to patients without symptoms" — but wouldn't you want the patient to have the choice?