Associations of Serum Serotonin Levels with 12-week and 12-month Remission in Patients with Depressive Disorders.

Objective: To investigate associations between baseline serum serotonin levels and short- and long-term treatment outcomes in outpatients with depressive disorders in a naturalistic one-year prospective study design. Methods: Patients were recruited at a University hospital in South Korea from March 2012 to April 2017. At baseline, blood samples were obtained from 1,094 patients who received initial antidepressant monotherapy (Step 1). After the Step 1 treatment, further treatment steps (at least Steps 2-4) could be administered every 3 weeks during the acute treatment phase (3, 6, 9, and 12 weeks; n = 1,086), and every 3 months during the continuation treatment phase (6, 9, and 12 months; n = 884). In cases showing an insufficient response or intolerable side effects, patients were asked to choose whether to remain at the current step or enter the next treatment step, with alternative strategies including switching, augmentation, combination, and a mixture of these approaches. Remission was defined as a Hamilton Depression Rating Scale score of ≤ 7. Results: The remission group had significantly higher baseline serum serotonin levels among patients who received Step 1 monotherapy in both acute and continuation treatment phases. These associations remained significant after adjustment for relevant covariates. No associations were found with any other treatment steps. Conclusion: Baseline serum serotonin levels may be used as a biomarker for predicting short- and long-term treatment outcomes in antidepressant monotherapy-treated patients with depressive disorders in a real-world clinical setting.