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Rationale: This test has been used as an aid in the diagnosis of polyuric disorders.1 Individuals with diabetes insipidus (DI), due to either hypothalamic (HDI) or nephrogenic disease (NDI), cannot conserve free water. When these patients are deprived of water for extended periods they will lose weight due to dehydration to a greater extent than healthy controls. This water loss is accompanied by hypernatremia and an increase in plasma osmolality. Normal individuals and patients with psychogenic polydipsia will not lose >3% of their body weight during the water deprivation test and will maintain serum sodium and osmolality levels within normal limits. The urine osmolality of patients with DI remains low, often less than that of plasma whereas subjects that do not have DI will produce concentrated urine (osmolality typically between 300 and 400 mOsm/kg). Some patients with psychogenic polydipsia fail to produce concentrated urine unless the deprivation is prolonged. Patients with NDI show high ADH levels as the plasma osmolality exceeds 300 mOsm/kg while patients with HDI have low or normal levels.
Administration of ADH in the second phase of the test can be used to identify the cause of DI. ADH administration to patients with HDI will cause an increase in urine osmolality >10% and will fail to produce this level of urine concentration in patients with NDI.1
Protocol (Phase 1): The test should be initiated at 10 PM at which time serum and urine specimens are collected for the determination of sodium and osmolality. The patient should also be weighed at this time. No oral intake is allowed until the test is terminated. At 6 AM, the following morning, the patient should be weighed again. Weight should be measured and urine should be collected hourly for measurement of volume and determination of osmolality. Once urine osmolality becomes stable (a change <30 mOsm/kg for two consecutive hours) specimens are collected for serum sodium, osmolality, and ADH levels.1
Protocol (Phase 2): Five units of aqueous vasopressin (ADH) is given subcutaneously, and urine osmolality is measured one hour later. The test can then be terminated.1
Orderable Tests: See Comprehensive List of Procedures section for individual test information.
Note: This test should not be performed if the patient's initial serum osmolality is >295 mOsm/kg. The test should be halted if the patient loses >5% of his or her body weight.1
1. Demers LM. Pituitary function. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry.3rd ed. Philadelphia, Pa: WB Saunders; 1999:1470-1499.