Definition
Polyuria is a urine output of more than ~3 litres per day. A short, structured differential makes it manageable.
Causes
Osmotic diuresis
- Diabetes mellitus (glycosuria) — the commonest cause
- Other osmotic loads (e.g. post-obstructive/recovery diuresis, high urea)
Water diuresis (dilute urine)
- Diabetes insipidus — cranial (ADH deficiency) or nephrogenic (ADH resistance)
- Primary polydipsia (excess water intake)
Electrolyte and drug causes
- Hypercalcaemia, hypokalaemia (impair urinary concentration)
- Diuretics, lithium
- Chronic kidney disease (loss of concentrating ability)
Approach
- Check glucose, U&E, calcium and urine osmolality
- If a dilute urine persists with a water diuresis, perform a water deprivation test (± desmopressin) to identify diabetes insipidus vs primary polydipsia
MRCP-specific traps
- Diabetes mellitus (osmotic) is the commonest cause — check glucose first.
- Hypercalcaemia and hypokalaemia cause a nephrogenic concentrating defect.
- A dilute urine that concentrates on water deprivation indicates primary polydipsia, not DI.
Summary
Polyuria (>~3 L/day) sorts into osmotic diuresis (diabetes mellitus, recovery diuresis), water diuresis (diabetes insipidus, primary polydipsia) and electrolyte/drug causes (hypercalcaemia, hypokalaemia, diuretics, lithium). Check glucose, U&E, calcium and urine osmolality; use a water deprivation test to pin down a water diuresis.
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