Are PET scans helpful in diagnosing dementia?

This is a good post in The Geriatrician, a blog (  The post provides an overview of how a geriatrician thinks about the value of PET scans in diagnosing dementia. Apparently there is very little value so this geriatrician has sent only one to get a PET scan, despite the fact that half of his patients have dementia.

When the author says “PET scans,” he is referring to “FDG PET scans.”  I rather doubt his bottom-line would be any different if he were talking about amyloid PET scans.

The crux of the argument has to do with sensitivity , specificity, positive predictive value, and negative predictive value – statistical terms.  He refers people to Google and Wikipedia to look up definitions of these terms.  He also points to a “handy dandy calculator,” found at

The geriatrician says:

“[According] to the Alzheimer’s Association, PET scans to make the diagnosis of dementia are 95% sensitive and 75% specific.  Sensitivity means that if someone has dementia, the test will pick it up. Specificity means that the test doesn’t pick up other things like depression.  … While the sensitivity seems great, the specificity is the achilles heel.”

In the blog post, the geriatrician goes through the math.  In summary, he says:

“I would say that the test is useful as a rule out type of test for those who have an intermediate or low suspicion.  Not so much to make the diagnosis.  I wouldn’t be comfortable telling someone they have a fatal neurodegenerative disease when I have a 20% chance of being wrong.  Or even 8% chance.  The second point is that the test is only useful in the setting of a clinical suspicion.  You can see how the characteristics of the test change depending on clinical suspicion.  However when people talk about PET scans, they imply that maybe it would be useful BEFORE a person has clinical symptoms.  It’s not there yet. Maybe the new Amyloid PET scans but not the tagged glucose pet scans.  After going through the math, this is why I don’t use PET scans.  I think it’s more useful to hone my clinical skills than use a test to compensate for poor clinical skills.”

Read the full blog post here:

Are pet scans good enough to diagnose dementia?
The Geriatrician
Sunday, November 16, 2014