Science Friday Why Aren’t There Biomarkers For Mental Illness?
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Feb 23, 2026 
John Krystal, a Yale psychiatrist and neuroscience researcher, explains why clear biological tests for depression, PTSD, and other disorders remain elusive. He discusses the brain’s complexity, limits of tissue sampling, overlaps across diagnoses, and promising tools like imaging, blood tests, genetics, and behavioral data. He also considers whether diagnostic manuals are ready to include biomarkers.
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Brain Tissue Limits Make Biomarkers Hard
- Biomarker search is hard because we can't sample brain tissue directly.
- Researchers analyze donated postmortem brains to identify molecular signatures of specific cells and circuits related to psychiatric symptoms.
PTSD And Depression Share Symptoms But Differ Molecularly
- Different psychiatric disorders show overlapping symptoms but distinct molecular signatures.
- Yale/VA analyses found PTSD and depression overlap yet have important molecular differences despite shared depressive symptoms.
Diagnostic Heterogeneity Masks Biological Subtypes
- Diagnostic heterogeneity magnifies biomarker challenges because one diagnosis can encompass many presentations.
- DSM‑5 allows ~270 ways to meet major depression criteria, implying many biologically distinct subtypes.