Salt Wasting: The Diagnosis That Looks Like SIADH But Requires the Opposite Treatment

Same low osmolality. Same concentrated urine. Same high urine sodium. A clinician scanning those results would call it SIADH without hesitating — and then reach for fluid restriction. In salt wasting, that's exactly the wrong move.This episode covers three salt wasting syndromes where the confusion with SIADH causes real harm. First, cerebral salt wasting after subarachnoid hemorrhage — how to distinguish it from SIADH using volume status, fractional excretion of phosphorus, and the saline challenge, which functions as both a diagnostic test and a therapeutic intervention. Second, cisplatin-induced renal salt wasting — a tubular toxicity that causes massive ongoing sodium losses (up to 400 mEq/day) in oncology patients, and why the combination of polyuria, hypovolemia, and a high urine sodium should never be attributed to SIADH. Third, vomiting-associated hyponatremia — why the urine sodium lies in the setting of metabolic alkalosis, why the urine chloride tells the truth, and how bicarbonaturia drags sodium into the urine even in a truly volume-depleted patient.The three-question framework at the close gives you a systematic way to work through any case where the urine sodium is high and SIADH is on your differential.Next episode: water intoxication, exercise-associated hyponatremia, and the low solute syndromes — where ADH is suppressed and the kidneys are doing everything right, but the patient is still hyponatremic.