Laboratory findings:
(1) plasma osmolality < 275 mOsm per kg water (some use < 270)
(2) urinary osmolality > 100 mOsm per kg water AND normal renal function
(3) clinical euvolemia
(3a) absence of hypovolemia (decreased skin turgor, hypotension, tachycardia, dry mucous membranes)
(3b) absence of hypervolemia (ascites, peripheral edema)
(4) elevated urinary sodium excretion (> 40 mmol/L) AND normal water and salt intake
where:
• A high urine sodium excretion is common in SIADH but may be absent if the patient undergoes severe salt and water restriction.
Exclusions:
(1) hypothyroidism
(2) hypocorticalism
(3) diuretic use
(4) pseudohyponatremia
Supportive findings:
(1) high serum antidiuretic hormone activity
(2) inability to excrete >= 90% of a water load test of 20 mL per kg body weight within 4 hours
(3) failure to dilute (reduce urine osmolality) following a water load test
