Water Deprivation Test Interpreter
Water Deprivation Test Interpreter
The indirect water deprivation test measures the kidney’s concentrating ability and infers the hormone from it. It is right about 70 to 77% of the time, and it is at its worst on exactly the question it is usually asked — partial AVP deficiency against primary polydipsia. This page reads the osmolalities honestly, including when they cannot be read at all.
Water deprivation test
Osmolalities → interpretationPlasma osmolality 301 mOsm/kg and urine osmolality 520 mOsm/kg after deprivation; urine 610 mOsm/kg after desmopressin
What the test measures, and what it infers
Urine < 300 with plasma > 300 mOsm/kg → diabetes insipidus
then desmopressin: rise > 50% → AVP deficiency · rise ≤ 50% → AVP resistance
Urine 300–800 mOsm/kg → indeterminate, and no rule exists for it
- 8 hours of observed deprivation
- weight, blood pressure and pulse hourly; plasma osmolality and sodium every two hours. Stop for more than 3% weight loss, or a serum sodium above the normal range at 146 to 150 mmol/L. Unobserved thirsting is not a test, because the commonest confounder is the patient drinking
- > 300 mOsm/kg plasma
- the stimulus. Without it a dilute urine has not been shown to be inappropriate, which is why this page refuses to interpret a result taken before the plasma osmolality has risen
- desmopressin 2–4 µg
- intravenously, intramuscularly or subcutaneously, with urine osmolality hourly for one to two hours afterwards. The comparison is against the pre-desmopressin urine osmolality, not against an absolute value
- > 50% rise
- the classic separation of complete cranial from complete nephrogenic diabetes insipidus. It works cleanly only for the complete forms; partial deficiency and partial resistance both give intermediate rises
- 70–77% overall accuracy
- what the test achieves against a final diagnosis. 76.6% (95% CI 68.9–83.2) in Fenske's prospective comparison, 70 to 77% in the 2025 JCEM approach paper, against 96.5% for hypertonic saline-stimulated copeptin in the same patients
Worked example
Plasma osmolality 301 mOsm/kg and urine osmolality 520 mOsm/kg after deprivation; urine 610 mOsm/kg after desmopressin
520 mOsm/kg is below 800, so concentrating ability is not intact and diabetes insipidus is not excluded
The plasma osmolality did reach 301 mOsm/kg, so the deprivation was adequate and the result can be read
But 520 is not below 300 either, so this is not the dilute-urine pattern that defines diabetes insipidus
The desmopressin response is 610 ÷ 520 = 1.17, a rise of 17% — not the more than 50% that would indicate complete cranial disease, and not obviously the flat response of nephrogenic disease either
→ indeterminate. Partial AVP deficiency, partial AVP resistance and primary polydipsia all produce exactly this, and so does medullary washout in a patient whose hormone is fine. This is the commonest outcome of the test and the reason stimulated copeptin replaced it
The four patterns the test is trying to separate
| Diagnosis | Urine osmolality after deprivation | After desmopressin | Copeptin |
|---|---|---|---|
| Complete AVP deficiency (cranial DI) | < 300 mOsm/kg | Rises by more than 50% | ≤ 4.9 pmol/L after hypertonic saline |
| Complete AVP resistance (nephrogenic DI) | < 300 mOsm/kg | Rises by 50% or less | > 21.4 pmol/L at baseline, no stimulation needed |
| Partial AVP deficiency | 300–800 mOsm/kg, incomplete | Rises, but variably | ≤ 4.9 pmol/L after hypertonic saline |
| Primary polydipsia | 300–800 mOsm/kg, often incomplete | Little further rise | > 4.9 pmol/L after hypertonic saline |
Why this test has been superseded
| Water deprivation test | Hypertonic saline-stimulated copeptin | |
|---|---|---|
| What is measured | The kidney's concentrating response | The pituitary's hormone output, directly |
| Overall diagnostic accuracy | 76.6% (95% CI 68.9–83.2) | 96.5% (95% CI 92.1–98.6) |
| Partial AVP deficiency vs primary polydipsia | Especially poor | The question it was built for |
| Duration | Eight hours of observed deprivation, then one to two more | Until the sodium reaches target, typically two to three hours |
| Confounded by | Surreptitious drinking; medullary washout; incomplete deprivation | Nausea and vomiting, which raise copeptin independently |
| Burden on the patient | Prolonged thirst under supervision | An infusion with sodium monitoring; more comfortable as arginine, less accurate |
An indirect test, and what it is indirect about
The water deprivation test does not measure vasopressin. It withholds water for about eight hours, watches the plasma osmolality rise and the urine osmolality follow or fail to, then gives desmopressin and watches again. From those two observations it infers whether vasopressin is being secreted and whether the kidney can respond to it. That inference was the best available for fifty years, and it works cleanly at the extremes: a urine osmolality above 800 mOsm/kg excludes the diagnosis, and a urine below 300 against a plasma above 300 establishes it.
The difficulty is everything in between, which is where most patients sit. Partial AVP deficiency, partial AVP resistance and primary polydipsia all concentrate the urine incompletely, and the three cannot be told apart by how incompletely. Worse, chronic polyuria of any cause — including polyuria driven purely by drinking — washes solute out of the renal medulla and blunts concentrating ability in a person whose vasopressin axis is entirely intact. So a mediocre concentrating response is not even specific to the hormone. Measured against a final diagnosis made months later, the test is right in about 70 to 77% of cases, and its accuracy is worst on precisely the distinction it is usually requested for.
What changed is that the hormone became measurable. Copeptin, the C-terminal fragment cleaved from the same pro-vasopressin precursor and released in equimolar amounts, is stable, abundant and measurable on a routine automated platform. Fenske's 2018 study ran both tests in the same patients and reported 96.5% accuracy for hypertonic saline-stimulated copeptin against 76.6% for water deprivation; arginine-stimulated copeptin, established in the Lancet the following year, offers a gentler protocol that needs no sodium monitoring, at the cost of a wide indeterminate zone. Hence the honest summary: the water deprivation test is no longer the test of choice for hypotonic polyuria, and where stimulated copeptin is available it should be used instead.
Where the deprivation test is still performed, three practical points decide whether the result is usable at all. The deprivation must be observed, because surreptitious drinking is the commonest reason a test fails and the patients most likely to do it are the ones the test is being used to diagnose. It must be stopped for more than 3% weight loss, which matters because a patient with complete cranial diabetes insipidus can become dangerously dehydrated inside a few hours. And the plasma osmolality must actually rise above 300 mOsm/kg before the urine result means anything — a dilute urine measured before the stimulus has been delivered is not evidence of anything, which is why this page refuses to interpret one.
Frequently asked questions
How is the water deprivation test performed?
Water is withheld under observation for about eight hours. Weight, blood pressure and pulse are recorded hourly and the plasma osmolality and serum sodium every two hours; the test stops for more than 3% weight loss or a serum sodium of 146 to 150 mmol/L. Desmopressin 2 to 4 µg is then given intravenously, intramuscularly or subcutaneously, with urine osmolality measured hourly for one to two hours afterwards.
What urine osmolality excludes diabetes insipidus?
800 mOsm/kg or more. An early-morning urine osmolality of 800 to 1200 mOsm/kg excludes the diagnosis on a single specimen, without any deprivation at all, which is worth checking before booking a formal test.
How does desmopressin separate cranial from nephrogenic diabetes insipidus?
By supplying the hormone. If the urine osmolality rises by more than 50% after desmopressin, the kidney could always respond and the problem was a lack of vasopressin — cranial diabetes insipidus, now called AVP deficiency. A rise of 50% or less means the kidney cannot respond to hormone that is plainly available, which is AVP resistance.
Why is the water deprivation test considered unreliable?
Because it is indirect and its accuracy is about 70 to 77%. It measures the kidney's concentrating response and infers the hormone, so anything else that blunts concentration — chronic polyuria washing out the medullary gradient, incomplete deprivation, drinking during the test — produces the same picture. It is at its weakest separating partial AVP deficiency from primary polydipsia, which is the question it is usually asked.
What has replaced the water deprivation test?
Stimulated copeptin. Hypertonic saline-stimulated copeptin achieved 96.5% accuracy against 76.6% for water deprivation in the same patients, using a cut-off of 4.9 pmol/L at a plasma sodium of 150 mmol/L. Arginine-stimulated copeptin is gentler and needs no sodium monitoring but is less accurate overall.
Can the test be interpreted if the plasma osmolality has not risen?
No. The diagnostic criterion is a urine osmolality below 300 mOsm/kg with a plasma osmolality above 300, and without the second half a dilute urine only shows the patient is not yet dehydrated. Continue the deprivation, or repeat the test — and check that the patient was not drinking.
Related calculators
References
- Fenske W, Refardt J, Chifu I, et al. A copeptin-based approach in the diagnosis of diabetes insipidus. N Engl J Med. 2018;379(5):428–439. — water deprivation test accuracy 76.6% (95% CI 68.9–83.2) against 96.5% for hypertonic saline-stimulated copeptin in the same patients.
- Newell-Price J, Drummond JB, Gurnell M, et al. Approach to the patient with suspected hypotonic polyuria. J Clin Endocrinol Metab. 2025;110(2):e506–e514.
- Kalra S, Zargar AH, Jain SM, et al. Diagnostic tests for diabetes insipidus. In: Endotext. MDText.com; NCBI Bookshelf NBK537591. — protocol, the 800–1200 mOsm/kg exclusion, the 3% weight-loss and 146–150 mmol/L stopping rules, desmopressin 2–4 µg and the >50% rise.
- Winzeler B, Cesana-Nigro N, Refardt J, et al. Arginine-stimulated copeptin measurements in the differential diagnosis of diabetes insipidus: a prospective diagnostic study. Lancet. 2019;394(10198):587–595.
- Timper K, Fenske W, Kühn F, et al. Diagnostic accuracy of copeptin in the differential diagnosis of the polyuria-polydipsia syndrome: a prospective multicenter study. J Clin Endocrinol Metab. 2015;100(6):2268–2274.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.