Which of the following criteria fit with a diagnosis of syndrome of inappropriate antidiuretic hormone secretion (SIADH)?
Hyperkalaemia
Hypernatraemia
Serum hyperosmolality
Urine osmolality greater than 100milliosmoles/kg
Urine sodium less than 20mmol/litre
Explanation:
SIADH results in excess ADH production with subsequent excess water reabsorption at the distal tubules. The resulting picture is one of dilute serum and concentrated urine. It is characterised by the following essential criteria:
normal renal, adrenal and thyroid function
no recent use of diuretics
clinical euvolaemia
decreased serum osmolality (less than 275 milliosmoles/kg of water)
increased urine osmolality (more than 100 milliosmoles/kg of water) in the context of hypotonic serum.
Hypernatraemia does not fit as the patient should be hyponatraemic in SIADH. The serum potassium level is typically within normal limits.
Urine sodium less than 20 mmol/litre suggests a dilute rather than concentrated urine. The urine sodium is typically >20 mmol/litre in SIADH.
Further Reading:
Ellison D, Berl T. The syndrome of inappropriate antidiuresis. N Engl J Med. 17 May 2007;356(20):2064-72.