Stratifying the Inflamed Endotype in Difficult-to-Treat Depression: A Roadmap from Biomarkers to Precision Immunopsychiatry.

Difficult-to-treat depression (DTD) extends beyond pharmacological non-response to encompass persistent symptoms, disability, and functional impairment despite optimal therapeutic strategies. This opinion advances a paradigm-shifting thesis: a biologically distinct inflammatory endotype exists within DTD, characterized by low-grade systemic inflammation (hs-CRP ≥3 mg/L), where treatment failure reflects a fundamental mechanistic mismatch between the pathogenic driver (neuroinflammation) and conventional monoaminergic interventions. We propose a precision stratification workflow: screen with hs-CRP, confirm and phenotype using a parsimonious cytokine panel (IL-6, TNF-α, IL-1β), and align interventions to underlying biology. Therapeutic options include targeted anti-inflammatory agents, neuromodulation, psychotherapy, lifestyle modifications, and digital monitoring. Assessment should prioritize patient-centered outcomes: functioning, quality of life, and cognition alongside symptom reduction. We outline pragmatic implementation within phased-care and public health systems, emphasizing equitable biomarker access and biomarker-enriched clinical trials. Reframing DTD through neuroinflammation transforms clinical heterogeneity from an obstacle into an opportunity: biomarker-guided precision care that matches mechanism to intervention, improving outcomes for patients whose depression has proven refractory to standard approaches.