Hyponatremia algorithm from European guideline:
Check these three labs: Posm, Uosm and UNa
- Posm confirms hyponatremia is real. (280-295 mOsm/Kg)
- Uosm tells you if urine is being concentrated (ability of kidneys to retain water [>100 mOsm/Kg]).
- UNa tells you if urine has excessive spillage of salt (ability of kidneys to reabsorb Na [<40 mmol/L]).
A normal kidney can concentrate urine (high UOsm) and can reabsorb Na (low UNa), therefore:
- HIGH and LOW means kidney is doing its job and there a problem with some other organ (edematous states: CHF, cirrhosis, *nephrotic syndrome).
- LOW and HIGH (complete opposite) means the kidneys are damaged (acute renal failure, ATN, post obs diuresis).
- LOW and LOW means too much intake and urine is dilute and bland (beer potomania, polydipsia, too much IV fluids). That leaves,
- HIGH and HIGH for the miscellaneous stuff – SIADH, CSW, endocrinopathies, HCTZ use.
Other tests to complete work-up include: TTE, TSH, LFTs/liver US, cortisol level, 24h urine Pr-.
Water restrict edematous states and SIADH. Replete salt wasting with normal saline.
SIADH and CSW may look the same in the labs but differs with respect to volume status and urine output (UO). SIADH is euvolemic, with normal UO while CSW is hypovolemic with high UO.
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Hoorn, Ewout J., and Robert Zietse. 2017. “Diagnosis And Treatment Of Hyponatremia: Compilation Of The Guidelines”. Journal Of The American Society Of Nephrology 28 (5): 1340-1349. doi:10.1681/asn.2016101139.