A 52-year-old woman with recurrent major depressive disorder is referred for suicide risk evaluation after expressing passive death wishes. She denies active suicidal ideation or intent. Which investigation is most appropriate to objectively identify neurobiological markers of suicide risk in this patient?

Explanation

## Neurobiological Markers of Suicide Risk **Key Point:** Cerebrospinal fluid 5-hydroxyindoleacetic acid (CSF 5-HIAA), the major metabolite of serotonin, is the most well-established biological marker of suicide risk. Low CSF 5-HIAA levels are associated with violent suicide attempts and completed suicide, independent of psychiatric diagnosis. ### Evidence for CSF 5-HIAA as a Suicide Marker **High-Yield:** CSF 5-HIAA is decreased in: - Violent suicide attempters (especially violent methods: hanging, jumping, self-immolation) - Completed suicides (post-mortem studies) - Impulsive aggression and poor impulse control - Across multiple psychiatric diagnoses (depression, bipolar disorder, schizophrenia, personality disorders) This suggests a **serotonergic dysfunction** underlying suicide risk, independent of mood disorder severity. ### Clinical Interpretation **Mnemonic: "LOW 5-HIAA = LETHAL LETHALITY"** — Low CSF serotonin metabolite = increased risk of violent, lethal suicide attempts. The association is: - Specificity: ~80% for predicting future violent suicide attempts - Sensitivity: ~60% (not all suicides have low 5-HIAA) - Predictive value: Remains abnormal even after remission of depression, suggesting a trait marker **Clinical Pearl:** CSF 5-HIAA is particularly useful in identifying patients at risk for *violent* suicide methods, even if current ideation is passive. This patient's passive ideation does not exclude high biological risk. ### Why Other Investigations Are Inadequate | Investigation | Relevance to Suicide Risk | |---|---| | Dexamethasone suppression test (DST) | Marker of HPA axis dysfunction in depression; not specific for suicide risk; poor sensitivity/specificity | | PET scan | Research tool only; not clinically validated for suicide risk stratification; expensive, not routine | | TSH/Free T4 | Screen for hypothyroidism (can worsen depression); not a suicide risk marker | **Warning:** Do not confuse DST (HPA axis marker) with suicide risk assessment. DST abnormality suggests depression severity, not suicide risk. ### Limitations and Clinical Context - CSF 5-HIAA requires **lumbar puncture** (invasive), so it is used primarily in research or high-risk inpatient settings - In routine clinical practice, suicide risk is assessed via **structured interviews** (C-SSRS) rather than CSF markers - However, in exam questions about *objective neurobiological markers*, CSF 5-HIAA is the evidence-based answer **Tip:** Distinguish between *clinical assessment tools* (C-SSRS, structured interviews — first-line) and *research/biological markers* (CSF 5-HIAA, neuroimaging — for understanding pathophysiology and high-risk phenotypes).