What you'll need: serum sodium, urine sodium, urine osmolality1, total urine daily volume.
Once again, there are three main questions to answer in working up the patient with hypernatraemia:
1. Is the hypernatraemia due to water loss or sodium gain? You can solve this riddle just by looking at the patient. If their extracellular fluid volume is expanded (e.g. oedema), then they've gained sodium, either because:
- you gave it to them (iatrogenic administration of hypertonic saline or hypertonic sodium bicarbonate) - look for a higher urine sodium concentration; or
- they have an excess or mineralocorticoid - look for hypertension, and a hypokalaemic metabolic alkalosis; urine sodium is variable.
You can now stop looking. However, if the extracellular fluid volume is anything but overloaded, then the hypernatraemia is due to water loss, and you must answer the second question.
2. Is this water loss renal or extrarenal? You judge this by looking at the urine results. If the loss is extrarenal (e.g. gastrointestinal water loss, or insensible water loss), then the kidneys will respond appropriately by excreting a small volume (~500 mls), hypertonic (more than 800 mosmol/kg) urine. Hooray! Have you found your cause of hypernatraemia yet? If not, then the kidneys must be to blame, and you must answer the third question.
3. Is the renal water loss due to a diuresis? Either an osmotic diuresis (e.g. flushing out masses of glucose after a diabetic ketoacidotic episode has been treated) or a diuretic will cause water to be lost in the urine, usually in excess of sodium. In either case, the daily urine osmole excretion rate will be high (more than 750 mosmol per day). Calculate this by multiplying the urine osmolality by the urine volume. For example, if the urine osmolality is 400 mosmol and the amount of urine passed in a day is 2.5 litres, then the daily urine osmole excretion rate will be:
400 ⨯ 2.5 = 1000 mosmol/day
If the excretion rate isn't high, then your patient sadly has diabetes insipidus. Further workup will include differentiating nephrogenic from central diabetes insipidus - for instance, by administering desmopressin.
Hopefully that wasn't too painful! In the next post, you'll have the opportunity to put your physiology to the test a bit...
1. In hypernatraemia, unlike hyponatraemia, you don't have to worry about other osmolytes (like glucose) getting in the way of your reasoning. You can safely assume that hypernatraemia is a hyperosmotic state, and so you don't need to pull a serum osmolality.