The detection of circulating BDNF levels has become an area of great interest in recent years due to their association with various neuropsychiatric disorders such as Parkinson's disease (Jiang et al., 2019), autism spectrum disorder (Liu et al., 2021), depression (Shi et al., 2020), and Alzheimer's disease (Ng et al., 2019). Here, BDNF is linked to major depressive disorder.