Last year, Medicare decided it would routinely pay for sophisticated imaging tests for Alzheimer’s disease called amyloid-PET (positron emission tomography) scans. The move will make it easier for people with early-stage memory loss to get tested and potentially become eligible for a class of new drugs that aim to slow the progression of Alzheimer’s.

However, many neuroscientists and imaging experts question whether amyloid PET can accurately identify early-stage Alzheimer’s or measure the benefits of these new drugs. Until now, Medicare paid for only one amyloid-PET scan over a patient’s’ lifetime, reflecting uncertainty about its value.

The new drugs, such as Leqembi, treat early-stage Alzheimer’s only, not any of the dozens of other forms of dementia. An incorrect diagnosis means people may receive the costly treatment even if they don’t have Alzheimer’s, thus getting no benefit and risking dangerous side effects.

Costs And Questions

It also means Medicare and consumers may spend billions of dollars on unreliable testing and ineffective and risky drugs. Each PET scan costs Medicare between about $700 and $1,200, and those being treated with the new drugs will need ongoing tests. A Medicare beneficiary’s cost for each test is about $300. An uninsured patient can expect to pay up to $3,000.

The annual cost of Leqembi itself is about $26,500. While Medicare pays most of the cost, beneficiaries must pay 20 percent, or about $5,000.


Some experts, such as Poul F. Høilund-Carlsen of Denmark’s Odense University Hospital, say a different kind of PET scan, called an FDG-PET, may be a more accurate way to identify early stage Alzheimer’s. Others recommend an older test, called a lumbar puncture, or spinal tap, that may be a better predictor of Alzheimer’s. But lumbar punctures are invasive and can cause headaches. And even they are imperfect diagnostic tools.

Researchers are exploring many alternatives. Some are studying whether they can use spinal taps to identify other Alzheimer’s biomarkers. Others are trying to develop new blood tests that more directly identify symptoms of Alzheimer’s, such as broken connections between brain cells. Or ways to image both AB and other brain material to better identify Alzheimer’s. Another group is looking at the value of a multi-day on-line cognitive test. But most alternatives remain experimental.

Because the new drugs are targeted only to those with early-stage Alzheimer’s or cognitive impairment that is expected to lead to Alzheimer’s, it is essential that a patient’s disease be correctly identified. Many researchers believe that Alzheimer’s is associated in some way with higher levels of a protein called Amyloid beta, or AB. They say if you find high levels of AB in patients’ brains, you can be sure they have Alzheimer’s disease, even if they show no symptoms.

A Crude Proxy

But the scans can’t see Alzheimer’s in the same way an X-Ray shows a broken bone. Instead, they measure AB and sometimes a related protein called tau. But many people with high levels of AB in their brains never will have symptoms of Alzheimer’s. And many with the disease do not have unusual levels of AB. Thus, testing for these proteins may be only a crude proxy for Alzheimer’s.

This sort of indirect diagnostic test is much more challenging, especially in an organ as complicated and poorly understood as the brain. As one imaging expert told me, testing for AB is “necessary but not sufficient” to diagnose Alzheimer’s.

Using AB as a proxy for Alzheimer’s creates a second problem. While new drugs such as Leqembi do clear AB from the brain, they only modestly slow progression of the disease. That raises more questions about both the drugs and amyloid-PET which, after all, measures amyloid but not brain function.

Critics say the third problem is that that amyloid-PET best identifies changes in parts of the brain that are not affected by early-stage Alzheimer’s, raising additional questions about its benefits.

Benefits, But For Whom?    

FDG-PET scans may more accurately identify amyloid beta in those areas that affect a patient’s ability to function. These scans are used in some developed countries to help identify Alzheimer’s. But while Medicare now pays for multiple amyloid-PET scans for Alzheimer’s and FDG-PET commonly is used to trace other diseases including some cancers, FDG-PET generally isn’t used in the US to diagnose Alzheimer’s.

Another problem: Sheena Shipley at the University of Colorado and colleagues found that physicians often misread the results of PET scans and diagnose Alzheimer’s that is not there.

Medicare’s decision to open the door to amyloid-PET scans almost certainly will result in more patients being diagnosed with Alzheimer’s and receiving these drugs. That will be good for clinics with PET scanners, for drugmakers, and, perhaps, for some patients. But it also will cost taxpayers billions of dollars with uncertain benefits for many other patients.