MPO and major depressive disorder: A commercially available test, using a combination of serum-based biomarkers, has been proposed to identify the probability of depression in a subject by giving an MDD score.41 The serum-based measures include alpha 1 antitrypsin, apolipoprotein CIII, myeloperoxidase, soluble tumor necrosis factor a receptor II, cortisol, epidermal growth factor, prolactin, resistin, and brain-derived neurotrophic factor.41 This study relies on protein biomarkers and specialized proprietary laboratories, where the MDD score is calculated.