What does a water deprivation test show?

Water Deprivation Test: Diagnostic Purpose and Interpretation

The water deprivation test is primarily used to differentiate between central diabetes insipidus, nephrogenic diabetes insipidus, and primary polydipsia by evaluating the kidney's ability to concentrate urine in response to water restriction and vasopressin administration. 1, 2, 3

Diagnostic Principles

The water deprivation test works by:

1. Inducing controlled dehydration to stimulate endogenous vasopressin (AVP) release
2. Measuring the kidney's response to this physiological stimulus
3. Evaluating response to exogenous desmopressin when administered after the dehydration phase

What the Test Shows

• In healthy individuals: Urine becomes concentrated (osmolality >800 mOsm/kg) in response to water deprivation
• In central diabetes insipidus: Minimal increase in urine concentration during dehydration, but significant increase after desmopressin administration
• In nephrogenic diabetes insipidus: Minimal increase in urine concentration during both dehydration and after desmopressin administration
• In primary polydipsia: Appropriate increase in urine concentration (>680-800 mOsm/kg) during dehydration, with minimal additional response to desmopressin 3

Specific Parameters Measured

1. Urine osmolality: Key parameter showing kidney's concentrating ability

   • Normal response: >800 mOsm/kg after water deprivation
   • Diagnostic threshold: >680 mOsm/kg has 100% sensitivity for primary polydipsia 3

2. Serum osmolality: Reflects systemic hydration status

   • Normal: 285-295 mOsm/kg
   • Dehydration: >300 mOsm/kg 4

3. Urine-to-serum osmolality ratio: Helps differentiate between different forms of diabetes insipidus 5

4. Plasma vasopressin (AVP) levels: May be measured but have limited discriminatory value between conditions 3

5. Copeptin levels: Emerging as a more stable marker of vasopressin activity

   • <2.5 pmol/L with plasma osmolality >290 mOsm/kg suggests central diabetes insipidus

   • 2.5 pmol/L suggests no diabetes insipidus 3

Clinical Significance

The water deprivation test is particularly valuable when:

• Differentiating between central diabetes insipidus, nephrogenic diabetes insipidus, and primary polydipsia in patients with polyuria (>3L/24h) 2
• Evaluating partial forms of diabetes insipidus that may be difficult to diagnose clinically 6
• Confirming the diagnosis when imaging and clinical features are inconclusive

Important Considerations

• Safety concerns: The test should be performed under medical supervision due to risk of severe dehydration
• Alternative approaches: Newer diagnostic methods like direct copeptin measurement after hypertonic saline infusion may eventually replace traditional water deprivation testing 1, 7
• Limitations: The test may not always clearly differentiate partial central diabetes insipidus from primary polydipsia 5
• Genetic testing: May be preferable to water deprivation testing in cases with strong clinical suspicion of congenital nephrogenic diabetes insipidus 1

Test Interpretation

• Urine osmolality >800 mOsm/kg after water deprivation: Suggests primary polydipsia (96% sensitivity, 100% specificity) 3
• Urine osmolality <300 mOsm/kg after water deprivation with significant increase after desmopressin: Suggests complete central diabetes insipidus 2
• Minimal response to both water deprivation and desmopressin: Suggests nephrogenic diabetes insipidus 8, 2

The water deprivation test remains a valuable diagnostic tool despite emerging alternatives, but should be performed in specialized centers with experience in managing potential complications of controlled dehydration.