The question patients and their families should be asking is, what does this test really offer?

Let’s be clear: it does not cure the disease or affect symptoms. At best it identifies what might be causing a patient’s forgetfulness. But, at worst, it can get that diagnosis wrong. According to the company’s own post-mortem study of 59 terminally ill patients, false positives in scans for the presence of amyloid were reported in up to 3 percent of cases, while up to 20 percent of cases resulted in a false negative: patients were diagnosed as not having amyloid and thus Alzheimer’s, even when they did.

To make things more confusing, while colloquially called an “Alzheimer’s test,” the scan doesn’t diagnose Alzheimer’s dementia — it only determines the presence of amyloid. And our understanding of the connection between the two is shaky. While everyone who has Alzheimer’s also has amyloid, not everyone with amyloid has Alzheimer’s dementia. Almost a third of cognitively normal elderly people have these protein clusters in their brains. On the scan they would light up. But those patients don’t have Alzheimer’s dementia and we don’t know how likely they are to develop it in the future. Imagine the anguish of that error.

In fact, the only thing the scan can do is confirm that a patient’s cognitive problems are not caused by Alzheimer’s.

Even if it could diagnose Alzheimer’s, it probably wouldn’t make much difference to patients. Currently, there are five drugs on the market that treat dementia. None can cure the disease. None can change the downward trajectory of cognition. At best, they can relieve the symptoms of patients with moderate to severe Alzheimer’s for about six to 12 months. And let’s not forget these drugs all have side effects like fatigue, dizziness and pain.

Even those who advocated the general approval of the scan never argued that it would help patients decide whether or not to use medication: these drugs are not preventive; they are taken only by people who already have advanced Alzheimer’s dementia. As one expert who is collaborating with Lilly put it: “Having an amyloid scan is hugely helpful not in determining drug therapy but for other things” — like planning.

But today, individuals 55 or younger are likely to live past 80, which means they stand a good chance of getting Alzheimer’s dementia. We don’t need an expensive test to tell us that we should all be planning for the possibility of Alzheimer’s — investing in long-term care insurance, exercising and staying socially active.

Alzheimer’s disease is scary. But that is no reason for society to waste a lot of money on a test that really doesn’t help. It is a reason for a lot more research on Alzheimer’s, including Medicare’s randomized trial to evaluate the effectiveness of the test. This research will take time, but there is no other path forward.