In a previous post, I indicated that the NYT article on the spinal-tap test for Alzheimer's contained insufficient data for readers to fully buy its supposedly super predictive ability. Reader Chris P. looked up the specificity, and found it rather lower than we would have thought. This doesn't mean the test is bad, it is just not "perfect".
Now, let us assume the test is good enough. Let's think about the implications.
There are two basic ways by which the spinal tap proteins could be related to Alzheimer's. Either that there is a direct causal path, or that some underlying process causes both the deposit of spinal tap proteins and Alzheimer's. The article mentions a "prevailing hypothesis" that amyloid and tau accumulation are necessary for Alzheimer's. That would be consistent with the first possibility.
There is a second possibility, which is that what we observe is only correlation, not causation. This could happen if some other mechanism causes Alzheimer's but that mechanism also creates spinal tap proteins.
The researchers reported that of all those known to have Alzheimer's, almost all of them have spinal-tap protein deposits. This raises the spectre of reversing the causal path. What if these protein deposits are a result of having Alzheimer's?
The point is that establishing the cause of a disease is no easy matter. In Chapter 2, I looked at tracing foodborne disease outbreaks; explaining other diseases are no less challenging.