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Rationale: This test has been used to help diagnose the syndrome of inappropriate antidiuretic hormone (SIADH).1 SIADH is a syndrome where the autonomous production of ADH interferes with the ability of the kidneys to excrete a water load normally. This results in a dilutional hyponatremia and low plasma osmolality. The water loading test can be useful in equivocal cases were SIADH is suspected and the patient has mild hyponatremia or a low normal plasma sodium level. When the water loading test is administered to healthy individuals, plasma osmolality will drop by >5 mOsm/kg and urine will become diluted (osmolality will drop to <100 mOsm/kg). Ninety percent of the water load is typically excreted within four hours by healthy individuals. Patients with SIADH excrete <90% of the water load within four hours and do not produce urine osmolality <100 mOsm/kg.
Protocol: The test is initiated two hours after the patient has eaten a light breakfast. Serum and urine osmolalities are measured. The patient is given water to drink (20 mL/kg) over a 15- to 30-minute period. Lightly salted crackers may be given with the water if needed. The patient should remain recumbent during the test. Collect samples for plasma and urine osmolality at the beginning of the water loading and hourly for the next four hours. Total urine output over the four hours should be measured.1
Orderable Tests: See Comprehensive List of Procedures section for individual test information.
Note: This test should not be performed in patients who are already significantly hyponatremic because of the danger that the water load will worsen the hyponatremia. Normal results exclude SIADH but abnormal results can occur in patients with glucocorticoid deficiency, hypothyroidism, and renal disease.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.