Peripheral biomarkers in major depressive disorders

Peripheral biomarkers would be useful tools for the daily clinical diagnosis, monitoring, and therapy of major depressive disorders (MDD). Although there are intensive investigations to identify specific biomarkers, applicable and validated tests are still not available. In this review, we provide an up-to-date summary of the most robust biomarker studies of MDD. There are accumulating results in this field primarily in relation to the hypothalamus-pituitary-adrenal (HPA) axis, the inflammatory system, and the neurogenesis/growth factors, but these data are heterogeneous and the investigated biomarkers are not specific for MDD. As a vicious circle, the discrepancy of biomarker studies’ results can be explained by the difficulties of phenotypic measurements, which should be improved by biomarker tests. Thus, more sophisticated phenotype definitions (e.g. subphenotypes, enodphenotypes, etc.) are needed for more accurate results. The development of standardized biomarker lab tests is possible in the near future, especially, as we now have more sensitive and high throughput detecting systems. The biomarkers would greatly help not only in clinical practice but also in drug development.