Water Deprivation Test and Polyuria Approach
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Direct answer
The water deprivation test is a supervised fast that measures the kidney's ability to concentrate urine, and it separates primary polydipsia, central diabetes insipidus and nephrogenic diabetes insipidus. Urine osmolality rises after deprivation in primary polydipsia and when ADH action is intact, but it stays dilute in diabetes insipidus. The desmopressin response then decides the cause, because central diabetes insipidus responds to it while nephrogenic disease does not.
What you must remember
- Polyuria threshold: urine output above about 3 litres per day in adults, which is confirmed before testing.
- Primary polydipsia: excessive water intake suppresses ADH, and urine concentrates normally during deprivation.
- Central diabetes insipidus: ADH deficiency, so urine stays dilute during deprivation but rises after desmopressin.
- Nephrogenic diabetes insipidus: ADH resistance in the collecting duct, so urine does not concentrate even with desmopressin.
- Lithium: the commonest drug cause of nephrogenic diabetes insipidus.
- Other nephrogenic causes: hypercalcaemia, hypokalaemia and demeclocycline, along with inherited AVPR2 or AQP2 defects.
- Osmotic diuresis: glycosuria in uncontrolled diabetes mellitus gives high urine osmolality, which is not diabetes insipidus.
- Response threshold: a rise in urine osmolality of more than about 50 per cent after desmopressin is the usual criterion for central disease.
Running the test and reading the response
The water deprivation test is a supervised investigation, not a home test, because it can cause dangerous dehydration and hypernatraemia. The steps below set out the sequence, and books differ on exact cut-offs, so the pattern of response matters more than any single number.
- Confirm the polyuria first: Urine output above about 3 litres per day with low urine osmolality is documented, and plasma sodium and osmolality are measured before fluids are stopped.
- Withhold fluids under supervision: Fluids are stopped with hourly weight, urine and plasma osmolality and sodium recorded, and the test is halted for significant weight loss or rising sodium.
- Read the deprivation result: Urine that concentrates well above plasma osmolality indicates intact ADH action and points to primary polydipsia.
- Give desmopressin if urine stays dilute: A standard dose of desmopressin is given, and urine osmolality is measured again over the following hours.
- Interpret the desmopressin response: Central diabetes insipidus shows a clear rise in urine osmolality, while nephrogenic disease shows little or no change.
- Plan the cause: Central disease needs pituitary imaging, while nephrogenic disease needs a review of lithium, calcium and potassium.
A response that is partial rather than absent suggests partial central deficiency, and the clinical picture guides the final label. Plasma sodium that is already high makes the test unsafe.
Where the examiner tries to catch you
Polyuria questions are built around the sequence of tests, so the traps usually involve giving the wrong hormone or misreading a normal-looking urine result.
- Trap: giving desmopressin to a patient with primary polydipsia. Correct: desmopressin raises water retention in a patient already drinking large volumes, risking water intoxication and hyponatraemia.
- Trap: assuming lithium-induced polyuria responds to desmopressin. Correct: lithium causes nephrogenic diabetes insipidus, which does not respond to desmopressin, and amiloride or a thiazide is used instead.
- Trap: calling polyuria diabetes insipidus without checking glucose. Correct: glycosuric osmotic diuresis in uncontrolled diabetes mellitus gives high urine osmolality and is a different picture.
Frequently asked questions
What urine volume defines polyuria?
Polyuria is generally defined as urine output above about 3 litres per day in adults. Water deprivation and urine osmolality are then used to separate its causes, such as primary polydipsia, central diabetes insipidus and nephrogenic diabetes insipidus.
What is the expected response to desmopressin in central diabetes insipidus?
Urine osmolality in central diabetes insipidus rises clearly after desmopressin, commonly by more than about 50 per cent, showing that the kidney can concentrate urine once ADH is supplied. This response distinguishes central from nephrogenic diabetes insipidus.
Why can desmopressin cause hyponatraemia in primary polydipsia?
Desmopressin increases water retention in primary polydipsia, where the patient is already drinking large volumes, so water intoxication and hyponatraemia can develop. This is the reason desmopressin is avoided in primary polydipsia.
Which drug is the commonest cause of nephrogenic diabetes insipidus?
Lithium is the commonest drug cause of nephrogenic diabetes insipidus, because it impairs the aquaporin-2 response in the collecting duct. Demeclocycline, hypercalcaemia and hypokalaemia are other recognised causes of this resistance to ADH.
How does uncontrolled diabetes differ from diabetes insipidus in polyuria?
Uncontrolled diabetes mellitus causes osmotic diuresis with high urine osmolality and glycosuria, whereas diabetes insipidus produces dilute urine with low osmolality. Checking blood and urine glucose at the outset separates the two conditions quickly.