Essential features

Decreased effective osmolality (<275 mOsm/kg of water)

Urinary osmolality > 100 mOsm/kg of water during hypotonicity

Clinical euvolemia

No clinical signs of volume depletion of extracellular fluid

No orthostasis, tachycardia, decreased skin turgor, or dry mucous membranes

No clinical signs of excessive volume of extracellular fluid

No edema or ascites

Urinary sodium > 40 mmol/liter with normal dietary salt intake

Normal thyroid and adrenal function

No recent use of diuretic agents

Supplemental features

Plasma uric acid < 4 mg/dl

Blood urea nitrogen < 10 mg/dl

Fractional sodium excretion > 1%; fractional urea excretion > 55%

Failure to correct hyponatremia after 0.9% saline infusion

Correction of hyponatremia through fluid restriction

Abnormal result on test of water load (<80% excretion of 20 ml of water per kilogram of body weight over a period of 4 hours), or inadequate urinary dilution (<100 mOsm/kg of water)

Elevated plasma AVP levels, despite the presence of hypotonicity and clinical euvolemia