Urine specific gravity ≤ 1.005 and urine osmolality < 200 mOsm/kg are the hallmarks of diabetes insipidus. Random plasma osmolality is generally > 287 mOsm/kg. Suspect primary polydipsia when large volumes of very dilute urine occur with plasma osmolality in the low-normal range. Polyuria and elevated plasma osmolality despite a relatively high basal level of ADH suggests nephrogenic diabetes insipidus.
With mild polyuria, water deprivation can begin the night before the test. With severe polyuria, water restriction is carried out during the day to allow close observation. All water intake is withheld, and urine osmolality and body weight are measured hourly. When two sequential urine osmolalities vary by less than 30 mOsm/kg or when the weight decreases by > 3%, 5 U of aqueous ADH or desmopressin are administered subcutaneously. A final urine specimen is obtained 60 minutes later for osmolality measurement.
Water deprivation test results may be misleading in patients with chronic primary polydipsia, who may experience partial washout of the medullary interstitial gradient and downregulation of ADH release. This would resemble nephrogenic diabetes insipidus, with an inability to concentrate urine.
On MRI, T1-weighted images of the healthy posterior pituitary yield a hyperintense signal. This signal is also invariably present in primary polydipsia. In patients with central diabetes insipidus, this signal is absent, except in a few children with the rare, familial form of the disorder. It is also absent in most patients with nephrogenic diabetes insipidus.
For more on the workup of diabetes insipidus, read here.
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Cite this: Romesh Khardori. Fast Five Quiz: Test Your Knowledge of Diabetes Insipidus - Medscape - May 30, 2018.