Fluid and electrolyte balance investigations
How to read serum and urine osmolality, the osmolar gap, spot urine sodium, chloride and potassium, and fractional excretions of sodium, urea, magnesium and phosphate, with worked examples and the traps (diuretics, timing, bicarbonaturia).
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Serum sodium reports water balance, not sodium balance. Osmolality (serum and urine) shows what ADH is doing.
- Spot urine electrolytes show what the kidney is doing in response to the blood. There is no fixed normal: a value is right or wrong only against the serum and volume state at that moment.
- Send urine before treatment. Saline, diuretics and steroids change the answer within hours.
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Sources
- Fluid and electrolyte disorders (Therapeutic Guidelines)AU/NZTherapeutic Guidelines Ltd
- RCPA Manual (pathology test interpretation)AU/NZRoyal College of Pathologists of Australasia
- Use of urine electrolytes and urine osmolality in the clinical diagnosis of fluid, electrolytes, and acid-base disordersKidney International Reports (Kamel KS, Halperin ML; 6:1211-24), 2021
- Clinical practice guideline on diagnosis and treatment of hyponatraemia (ESE, ESICM, ERBP)European Journal of Endocrinology (Spasovski G, et al.; 170:G1-G47), 2014 (no newer version at November 2026)
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Turnstile bot check on sign-in and signed-out feedback