Depression's biological fingerprint: a quest for objective diagnosis and personalized treatment

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: General criteria: * Participants aged 14-45 years. * Participants are able to understand the study procedures and provide written informed consent. For participants younger than 18 years, both the participant and their legal guardian must provide consent. Major Depressive Disorder (MDD) cohort: * Meet DSM-5 criteria for major depressive disorder (single or recurrent episode). * Have a baseline Montgomery-Åsberg Depression Rating Scale (MADRS) score ≥24 and 17-item Hamilton Depression Rating Scale (HAMD-17) score ≥18. Healthy Control (HC) cohort: \- Healthy participants without a current or lifetime diagnosis of major psychiatric disorders. Exclusion Criteria: For all participants: \- Severe or unstable medical conditions, pregnancy or breastfeeding, or other conditions considered unsuitable for study participation. For the MDD cohort: * Current or lifetime diagnosis of other major psychiatric disorders, including schizophrenia spectrum disorders, schizoaffective disorder, or bipolar disorder. * Substance use disorder within 12 months prior to screening. * Depression secondary to medical or neurological conditions. * Regular antidepressant treatment within 2 weeks prior to enrollment. * Electroconvulsive therapy (ECT), repetitive transcranial magnetic stimulation (rTMS), vagus nerve stimulation (VNS), or immunosuppressive therapy during the current depressive episode. * Current active suicidal plan or recent suicidal behavior considered unsuitable for participation. For the HC cohort: * Current or lifetime diagnosis of any psychiatric disorder. * Significant depressive, anxiety, manic, or psychotic symptoms. * Previous treatment with antidepressants, antipsychotics, or mood stabilizers. * Significant family history of major psychiatric disorders.