The test looks for a set of 10 proteins in the blood. I suspect that part is fairly reliable which means the theory that those proteins are markers for Alzheimer's is probably incorrect or incomplete.

Whether or not those 10 proteins are markers for Alzheimer's is not the issue at all. When you look for markers, the search goes this way: disease => chemicals. But when you start testing patients for markers, you're actually using the **inverse** of that relationship: chemicals => disease. If the disease is rare, false positives will vastly outnumber the number of patients really afflicted by the disease even though you test for the right markers.

There are 3 important percentages for disease tests:

- How many people are afflicted by the disease in the general population?

- How many **afflicted** people get positive result?

- How many **healthy** people get positive result (false positive)?

I'll give you an example. Let's say we have some test with perfect accuracy (100% of people afflicted by the disease get positive test) but the test has 10% rate of false positives. We'll test 1000 people.

If the disease afflicts 50% of the population, about 550 people will get positive result but about 50 of them are healthy.

If the disease afflicts 10% of the population, about 190 people will get positive result but about 90 of them are healthy.

If the disease afflicts 1% of the population, about 109 people will get positive result but about 99 of them are healthy.

If the disease afflicts 0,1% of the population, about 101 people will get positive result but about 100 of them are healthy.

Is the problem clear now?