water deprivation test
Summary
A diagnostic test for diabetes insipidus by withholding water and observing urine output and osmolality response to exogenous vasopressin. Differentiates central from nephrogenic DI.
Detail
The water deprivation test proceeds in two phases: First, the patient is water-restricted while urine output, plasma osmolality, and serum sodium are monitored. In DI, urine remains dilute despite rising plasma osmolality. Second, desmopressin is administered: in central DI, the kidneys respond with concentrated urine (>600 mOsm/kg), confirming ADH deficiency. In nephrogenic DI, there is minimal or no response, indicating renal resistance. The test must be performed carefully to avoid severe dehydration and is contraindicated in severe hypernatremia.
Sources
- First Aid for the USMLE Step 1
- Pathoma
- Harrison's Principles of Internal Medicine
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