MPO and major depressive disorder: In both pilot (n = 34 patients with major depressive disorder (MDD), n = 43 healthy controls) and replication studies (n = 36 MDD, n = 43 healthy controls), a serum assay comprising nine serum biomarkers distinguished MDD subjects from healthy controls with 91.7% sensitivity and 81.3% specificity; significantly elevated biomarkers for neuropsychiatric symptoms were the immunological molecules alpha 1 antitrypsin, myeloperoxidase, and soluble TNF-α receptor II [23].