In the present study, significantly increased BDNF in serum levels were found in OA participants with MDD, and OA participants who transformed into MDD after the follow-up also showed significantly increased serum BDNF levels at baseline and the end of follow-up, which suggested that serum BDNF may be used as a potential diagnostic and predictive biomarker to identify MDD in OA. The gene discussed is BDNF; the disease is major depressive disorder.