A 58-year-old man with small-cell lung cancer presents with confusion and lethargy. Serum sodium is 118 mEq/L. Clinical examination shows no signs of volume depletion or edema. Which investigation is most appropriate to confirm the suspected diagnosis of SIADH?

Explanation

## Diagnosis of SIADH: Role of Osmolality Measurements **Key Point:** The diagnosis of SIADH is confirmed by demonstrating inappropriately elevated urine osmolality (>200 mOsm/kg) in the setting of low serum osmolality (<280 mOsm/kg) and euvolemia. ### Diagnostic Algorithm for Hyponatremia ```mermaid flowchart TD A[Hyponatremia confirmed]:::outcome --> B{Assess volume status}:::decision B -->|Euvolemic| C[Measure serum osmolality]:::action C --> D{Serum osmolality?}:::decision D -->|Low <280| E[Measure urine osmolality]:::action E --> F{Urine osmolality?}:::decision F -->|High >200| G[SIADH diagnosis]:::outcome F -->|Low <100| H[Primary polydipsia]:::outcome B -->|Hypovolemic| I[Check urine sodium]:::action B -->|Hypervolemic| J[Check urine sodium]:::action ``` ### Why Osmolality Pair is Gold Standard | Parameter | SIADH | Primary Polydipsia | Hypovolemic Hyponatremia | |-----------|-------|-------------------|-------------------------| | **Serum osmolality** | Low (<280) | Low (<280) | Low (<280) | | **Urine osmolality** | High (>200) | Low (<100) | High (>300) | | **Volume status** | Euvolemic | Euvolemic | Depleted | | **Urine sodium** | >40 mEq/L | Variable | <20 mEq/L | **High-Yield:** Serum osmolality + urine osmolality is the **single most discriminatory pair** to differentiate SIADH from other causes of euvolemic hyponatremia. This is the investigation of choice for confirming SIADH. **Clinical Pearl:** In this case, the patient is euvolemic (no edema, no signs of volume depletion) with severe hyponatremia and a known malignancy — classic for SIADH. The osmolality pair will show low serum osmolality with inappropriately high urine osmolality, confirming the diagnosis. **Mnemonic: SIADH osmolality pattern = "Low serum, High urine"** — the kidneys are retaining water despite dilute plasma, which is the hallmark of SIADH.