BDNF and major depressive disorder: Moreover, Pearson correlation analysis showed that when BDNF serum levels were tested for simple linear correlations against genotypes, depression scores, gender, BMI, HbA1c, BUN, Cr and numbers of complications, significant inverse correlations were found with genotypes (r = −0.346, p < 0.01), depression scores (r = −0.416, p < 0.01) and HbA1c (r = −0.168, p < 0.05), indicating that low BDNF concentrations may be a pathogenic factor associated with both depression and T2DM.