Ng et al. (2019) reported that blood BDNF did not change in the MCI stage and only decreased in the late stage of AD. Not only are the conclusions of the difference in BDNF between AD patients and normal controls inconsistent, but there are also great differences in BDNF levels among different studies. Some studies reported that BDNF levels were very low (<5 pg/ml) (Nascimento et al., 2014; Wang et al., 2015), while others reported that BDNF levels were very high (>50 ng/ml) (Liu et al., 2015; Zhang et al., 2019). This evidence concerns the gene BDNF and Alzheimer disease.