The water deprivation test is useful in the workup of patients with polyuria under certain situations. The differential diagnosis of polyuria often comes down to the following: does the patient have diabetes insipidus (either central or nephrogenic), psychogenic polydipsia, or an osmotic diuresis (e.g., due to hyperglycemia for instance). The first two conditions (DI or polydipsia) are all characterized by polyuria with a dilute urine osmolarity, but how to distinguish between them? If the serum sodium is high (D.I.) or low (polydipsia) the answer may be easily apparent, but in the instance where serum sodium is within the normal range the water deprivation test can provide an answer.
However, it’s important to do this test in a controlled setting, as water restriction in a patient with D.I. can result in elevated serum sodium concentrations. Patients who undergo the water deprivation test should have the urine volume and urine osmolality every hour and plasma sodium concentration eveyr two hours once water deprivation begins. The test is continued until either:
(a) the urine osmolality reaches a normal value (e.g., above 600 mosm/kg, suggesting that both ADH secretion and response to ADH are intact).
(b) the urine osmolality is stable on two successive measurements despite a rising plasma osmolality, or
(c) the plasma osmolality is greater than 295-300 mosm/kg.
In the situations of either (b) or (c), exogenous ADH is administered and the urine osmolality and volume are further monitored.
In central D.I., exogenous ADH is predicted to lead to a rapid rise in urine osmolality: in complete D.I., the urine osm will more than double, while in partial central D.I. (which is more common) there will be an increase of at least 15% in the urine osm. Generally individuals with central D.I. are able to concentrate their urine osm > 300 mosm/kg.
In nephrogenic D.I., there is either no response to ADH (complete nephrogenic D.I.) or a blunted response to ADH (up to 45%), though patients are rarely able to concentrate their urine osm above 300 mosm/kg.
A more detailed protocol for the water deprivation test, approved by the Scientific Advisory Committee of the Diabetes Insipidus Foundation, Inc, can be found here