Normally a person excretes a water load within a few hours, with decreases in serum and urine osmolality. Persons with abnormal ADH secretion can only partially excrete the water load and do not show the expected decrease in osmolality in either serum or urine. This test is useful in patients with serum sodium and osmolality near normal or normal and who are suspected of having SIADH.


Procedure: An oral water load (20 mL/kg) is given over 15-20 minutes with the patient lying down. Plasma and urine osmolality are measured before and during the 5-6 hours after the water load. Urine output is collected and measured.


Restrictions: The serum sodium should be over 125 mEq/L and the patient should not be having symptoms of hyponatremia.




plasma osmolality

urine osmolality

water load excreted in 4 hours

plasma ADH at 2 hours after load


decrease >= 5 mOsm/kg

decreases to <= 100 mOsm/kg




decrease < 5 mOsm/kg

> 100 mOsm/kg

< 50%



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