Evaluation of Hypernatremia

The cause of hypernatremia can be suspected based on clinical and laboratory findings.

ECF volume expanded?	decrease in body weight from normal?	urine volume minimal and urine osmolality maximal?	Possible Cause
Y			sodium gain
N	N		water shift
N	Y	Y	nonrenal water loss
N	Y	N	(see next table)

where:

• ECF is extracellular fluid volume

• body weight decrease assumed if ratio current weight to usual weight is <= 0.988; this allows a loss of 0.9 kg (2 pounds) at 77.3 kg (170 pounds)

• maximal urine osmolality is taken here as > 1000 mOsm per kg water

urine osmolality very low?	respond to ADH administration?	after diuretic administration?	Possible Cause
Y	Y		central diabetes insipidus
Y	N		nephrogenic diabetes insipidus
N		Y	drug-induced
N		N	osmotic diuresis • glucose • urea • mannitol

where:

• very low urine osmolality is < 250 mOsm per kg water

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Purpose: To identify the cause of hypernatremia based on clinical and laboratory findings.