Polyuria differential
Formal Definition
A structured clinical framework for evaluating the differential diagnosis of polyuria — urine output exceeding 3 L/day (or >2.5 L/m²/day in children) — required for diagnostic reasoning in patients presenting with elevated urine output, thirst, or symptoms suggestive of water-balance disorders; the differential is conventionally organized around the four major mechanisms: (1) water diuresis (free water loss — diabetes insipidus central or nephrogenic, primary polydipsia), (2) solute diuresis (osmotic agents driving water loss — hyperglycemia in diabetes mellitus/DKA/HHS, mannitol, urea in post-obstructive diuresis or catabolic states), (3) impaired concentrating ability (chronic tubulointerstitial disease, sickle cell trait, lithium toxicity, hypercalcemia, hypokalemia), and (4) iatrogenic/pharmacologic causes (diuretics, lithium, demeclocycline, certain diabetes medications like SGLT2 inhibitors).
How It's Used on the Ward
"Polyuria workup" or "why is the patient peeing so much?" — systematic evaluation of a patient with high urine output; the framework splits into water diuresis (DI, water intake) vs solute diuresis (sugar in diabetes, urea, mannitol); first step is urine osmolality to determine dilute vs concentrated status.
Example
""48-year-old with 4 days of polyuria (8L/day reported), polydipsia, dizziness. Labs: glucose 480 mg/dL, BUN 28, Na 142, serum osm 312. Urine osm 480 mOsm/kg (concentrated urine). Diagnosis: osmotic polyuria from hyperglycemia (solute diuresis). Glucose drives water loss. Treatment: IV fluids, insulin, glucose correction. As glucose normalizes, urine output reduces. Contrast with: 34-year-old post-pituitary surgery with 8L urine/day, polydipsia, sodium rising (Na 152, serum osm 308, urine osm 95 mOsm/kg — dilute urine). Diagnosis: water diuresis from central diabetes insipidus.""
Clinical Context
Diagnostic approach: (1) Confirm polyuria — measured 24h urine output >3 L/day (formal) or patient-reported excess corroborated by clinical assessment. (2) Measure serum and urine osmolality simultaneously. (3) Categorize: solute diuresis = high urine osm (>600 mOsm/kg with intact ADH response), water diuresis = low urine osm (<300 mOsm/kg, dilute urine). (4) If solute diuresis — determine the solute (glucose, urea, mannitol). (5) If water diuresis — distinguish central DI (responds to desmopressin), nephrogenic DI (doesn't respond), primary polydipsia (responds to fluid restriction without desmopressin). Water deprivation test: incremental dehydration with serial osm measurements, then desmopressin challenge — central DI: urine osm rises substantially (>50% rise or >600 mOsm/kg). Nephrogenic DI: minimal response (<9% rise). Primary polydipsia: urine osm rises with dehydration without desmopressin. Plasma copeptin: emerging alternative to water deprivation test; high copeptin in nephrogenic DI and primary polydipsia, low in central DI. Treatment depends on underlying cause.