## Diagnostic Confirmation of SIADH ### Clinical Context The patient presents with the classic triad of SIADH: hyponatremia, low serum osmolality with inappropriately high urine osmolality, and euvolemia (no edema or volume depletion signs). Small-cell lung cancer is a well-known cause of ectopic ADH secretion. ### Why Plasma ADH Level is the Gold Standard **Key Point:** Simultaneous measurement of plasma ADH with paired plasma and urine osmolality is the definitive diagnostic test for SIADH. It demonstrates: - Elevated or inappropriately normal ADH in the setting of hypo-osmolality - Inability of the kidney to suppress ADH despite osmotic suppression stimulus ### Diagnostic Criteria for SIADH | Criterion | Finding | |-----------|----------| | Serum osmolality | < 275 mOsm/kg (hypo-osmolar) | | Serum sodium | < 135 mEq/L (hyponatremia) | | Urine osmolality | > 100 mOsm/kg (inappropriately concentrated) | | Urine sodium | Usually > 40 mEq/L | | Plasma ADH | Elevated or inappropriately normal for osmolality | | Volume status | Euvolemic (no edema, normal JVP) | | Thyroid & adrenal | Normal function | **High-Yield:** The key diagnostic finding is that ADH is NOT suppressed despite low plasma osmolality. In a healthy person, ADH would be undetectable when serum osmolality is < 280 mOsm/kg. ### Interpretation **Clinical Pearl:** The plasma ADH level should be measured when: 1. Serum osmolality is clearly low (< 275 mOsm/kg) 2. Urine osmolality is inappropriately high (> 100 mOsm/kg) 3. Patient is euvolemic An ADH level > 2 pg/mL in the setting of hypo-osmolality is diagnostic of SIADH. ### Reference Range - Normal plasma ADH: 1–5 pg/mL (varies by lab) - In SIADH: typically 5–50 pg/mL or higher, with failure to suppress below 2 pg/mL [cite:Harrison 21e Ch 286]
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