Study type: Blood collected in a prechilled lavender-top [EDTA] tube; related body system: Endocrine system.

Instructions regarding appropriate handling and transport of the specimen should be obtained from the testing facility prior to specimen collection. ADH is formed by the hypothalamus and stored in the posterior pituitary gland. ADH is important in the regulation of water reabsorption and is released in response to increased serum osmolality or decreased blood volume. ADH secretion exhibits diurnal variation; the highest levels occur at night, which helps prevent the need for urination during the night. When the hormone is active, water is reabsorbed by the kidneys into the circulating plasma instead of being excreted, and small amounts of concentrated urine are produced; in its absence, large amounts of dilute urine are produced. Although a 1% change in serum osmolality stimulates ADH secretion, blood volume must decrease by approximately 10% for ADH secretion to be induced. Psychogenic stimuli, such as stress, pain, and anxiety, may also stimulate ADH release, but the mechanism is unclear.

The ADH suppression, or water load, test is used in the differential diagnosis of SIADH from other causes related to sodium imbalances (e.g., adrenal insufficiency, excessive loss of sodium) or conditions that result in the development of edema (e.g., heart failure, myxedema, nephrosis). The suppression test is performed over a 6-hr period, in the fasting state, by administration of water at the initiation of the test (20 mL/kg of body weight to a maximum of 1,500 mL) followed by hourly measurements of serum and urine osmolality. The idea of osmolality being increased or decreased can be confusing in disease states because blood and urine levels vary in response to each other’s compensatory mechanisms in maintaining water balance. Normal serum osmolality is 275 to 295 mOsm/kg; urine osmolality is 250 to 900 mOsm/kg; and the normal ratio of serum osmolality to urine osmolality is 1:3. Urine and serum values should be evaluated together.

Normally we associate low values with small concentrations and high values with large concentrations. In the case of SIADH, urine osmolality values decrease in response to an abnormal increase in ADH secretion, telling the kidneys to retain water, concentrate the urine, and dilute the blood volume. A smaller urine osmolality number indicates highly concentrated urine, and a larger number indicates more dilute urine. Patients with SIADH excrete none or a very small amount of the water and have a more concentrated urine that measures a higher urine osmolality than expected if ADH was being properly suppressed (greater than 100 mOsm/kg, is an indication of the kidneys’ inability to dilute the urine), whereas patients with an imbalance of sodium or significant edema excrete some water and have a relatively more dilute or decreased urine osmolality than patients with SIADH (less than 300 mOsm/kg). Patients should be closely monitored and educated regarding the signs and symptoms of water intoxication, a potential complication of the suppression test.