BDNF and Alzheimer disease: A large number of studies suggest that blood BDNF could be used as a biomarker for AD diagnosis, prognosis, and treatment monitoring, but the survey results of blood BDNF levels in AD patients are very inconsistent (Bathina and Das, 2015; Song et al., 2015; Marta et al., 2020; Xie et al., 2020; Mori et al., 2021).