BDNF and major depressive disorder: Tests that can accurately and reproducibly indicate a medical state are generally referred to as biomarkers.10 To date, there are no biomarkers for depression; however, consistent with the high number of common comorbidities, depression is associated with changes in a wide range of clinical laboratory values, including increased proinflammatory cytokines,11,12,13,14 altered lipids,15,16,17 growth factors,18,19,20 and decreased brain-derived neurotrophic factor.21,22,23,24 For many of these physiological quantitative values, the underlying biological mechanisms are well understood.