Imaging Dementia-Mayo Clinic
How Accurate Are PET Scans for Alzheimer's Diagnosis?
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My father recently had a PET scan at a major hospital and research center in our area. He has been having memory and behavior issues for some while. The PET scan result states that he has findings consistent with early Alzheimer's type dementia (moderately decreased bilateral activity in the tempoparietal lobes). We meet with his neurologist in a month's time to discuss the findings. While I know that you cannot comment on individual cases, I ask: How accurate are PET scans nowadays? And is there some chance (what likelihood) of the results being wrong? Thank you in advance.
There is no test currently available that is diagnostic for Alzheimer’s disease. PET scans, MRIs, blood tests and others all may aid in making a diagnosis, but ultimately that is all they can do – aid in making a diagnosis. Results from a PET scan have to be taken in context with the patient’s history and performance on cognitive testing and are not part of the current recommended evaluation.
The type of PET scan used for dementia patients is also called PET-FDG. The first step is to inject a sugary substance (glucose) that contains weak radioactive markers into the patient’s vein. The substance then circulates throughout the body. The idea is that areas of the brain that are not working will not use the sugar, so those areas will not light up on the scan as well as areas that are using the sugar. For Alzheimer's, we are looking for reduced activity in the parietal and temporal lobes.
In studies of PET scans for diagnosing mild Alzheimer’s disease, the ability of these scans to accurately classify patients with and without early Alzheimer's disease varies quite a bit. A summary estimate is that PET is about 90 percent sensitive (meaning it would identify 90 out of 100 people with Alzheimer’s and miss 10) and 86 percent specific (meaning that 14 out of 100 people would be false positives, identified as Alzheimer’s without actually having the disease). Again, the estimates on these numbers vary quite a bit, and the accuracy is going to depend a lot on exactly how the test was done and the experience of the radiologist reading the results.
Currently, Medicare will only pay for PET scans if they ordered specifically to help distinguish AD from frontotemporal dementia (usually only seen in young patients). There are currently studies underway of a new kind of PET scan that will light up amyloid in the brain.
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