A 58-year-old man with small-cell lung cancer presents with confusion and lethargy. Serum sodium is 118 mEq/L. Physical examination shows no signs of volume depletion or edema. Urine osmolality is 650 mOsm/kg and urine sodium is 85 mEq/L. Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Context This patient presents with hyponatremia in the setting of malignancy with clinical and biochemical features suggestive of SIADH (syndrome of inappropriate antidiuretic hormone secretion). ## Diagnostic Approach to Hyponatremia **Key Point:** The first step in evaluating hyponatremia is to determine serum osmolality, which classifies hyponatremia into hypotonic, isotonic, or hypertonic categories. ### Why Serum Osmolality is the Investigation of Choice In suspected SIADH: - **Serum osmolality will be LOW** (< 280 mOsm/kg) despite low serum sodium - This confirms **hypotonic hyponatremia**, the hallmark of SIADH - The combination of: - Low serum osmolality - Inappropriately elevated urine osmolality (> 100 mOsm/kg) - Euvolemia (no edema, no signs of volume depletion) - Elevated urine sodium (> 40 mEq/L) ...establishes SIADH diagnosis **High-Yield:** SIADH is characterized by the paradox of **dilute serum with concentrated urine** — serum osmolality measurement confirms this paradox. ## Diagnostic Algorithm for SIADH ```mermaid flowchart TD A[Hyponatremia Na &lt; 135]:::outcome --> B[Measure serum osmolality]:::action B --> C{Osmolality &lt; 280?}:::decision C -->|Yes| D[Hypotonic hyponatremia]:::outcome C -->|No| E[Isotonic or hypertonic]:::outcome D --> F[Assess volume status]:::action F -->|Euvolemic| G[Check urine osmolality]:::action G -->|&gt; 100| H[SIADH likely]:::outcome H --> I[Exclude hypothyroidism<br/>and adrenal insufficiency]:::action ``` **Clinical Pearl:** The urine osmolality and urine sodium values given in the stem are already consistent with SIADH; serum osmolality measurement is the confirmatory test that completes the diagnostic triad. **Mnemonic: SIADH Diagnosis = SOD** — **S**erum osmolality (low), **O**smolality urine (high), **D**ilute plasma with concentrated urine.