BDNF isn't a good marker for specific brain diseases because many different conditions affect BDNF levels, but it might be useful for measuring memory problems that are common across different brain disorders.
Scientific Claim
BDNF is not a valid biomarker for neurodegenerative or neuropsychiatric diseases because its dysregulation is common to many pathological conditions, but it may be a marker specifically related to mnemonic symptoms across different disorders.
Source Excerpt
“We propose that, although BDNF may not be a valid biomarker for neurodegenerative/neuropsychiatric diseases because of its disregulation common to many pathological conditions, it could be thought of as a marker that specifically relates to the occurrence and/or progression of the mnemonic symptoms that are common to many pathological conditions.”
Evidence from Studies
Supporting Studies
Brain-Derived Neurotrophic Factor: A Key Molecule for Memory in the Healthy and the Pathological Brain
The abstract and conclusion explicitly state this proposal about BDNF's role as a biomarker. The language used is appropriately descriptive and cautious.