false premise. sugar in fruit is not good for you. fruit is nature tricking you into eating fiber and vitamins by luring you with sugar.

Because we know certain things appear when it is present. Think of a smoke detector, you have that to tell you there's a fire even though what it actually detects is smoke. Your smoke detector still works even if the smoke detector can't tell what made the smoke.

The point is right now the doctor has to just guess whether it's Alzheimer's or something else unless they can do expensive imaging tests and a spinal tap with expensive testing on the spinal fluid at a reference lab. An antibody test on blood would be way cheaper and easier to do at scale and allow earlier and more confident treatment to more people. This will become more important as treatment improves over time (hopefully).