Serum Osmolality / Osmolarity
Calculates estimated serum osmolarity from sodium, glucose, and urea. Compare with measured osmolality to detect an osmol gap (suggesting toxic alcohols or other unmeasured osmoles).
Score interpretation
Calculated osmolarity < 275 mOsm/kg: Hypo-osmolar state.
→ Evaluate for hyponatraemia. Check measured osmolality. If measured is normal with low calculated → pseudohyponatraemia.
Calculated osmolarity 275–295 mOsm/kg: Normal range.
→ Normal. If measured osmolality is significantly higher → osmol gap present (> 10 mOsm/kg). Consider toxic alcohols.
Calculated osmolarity 296–320 mOsm/kg: Mildly elevated. Consider dehydration, hyperglycaemia, hypernatraemia.
→ Investigate cause. IV fluids if hypovolaemic. Compare with measured osmolality.
Calculated osmolarity > 320 mOsm/kg: Significantly elevated. Seen in severe HHS, profound dehydration.
→ Urgent assessment. HHS: cautious fluid replacement. Measure serum osmolality. ICU/HDU if > 340 mOsm/kg.
Interpretation bands for the Serum Osmolality. Apply clinical judgement and local guidance.
References
- Bhave G, Neilson EG. Volume depletion versus dehydration: how understanding the difference can guide therapy. Am J Kidney Dis. 2011;58(2):302–309.
Related
Curated clinical cross-links plus same-class fallbacks.
- Potassium chloride with glucose and sodium chloride · IV fluid (mixed)
- Sodium chloride with glucose · IV fluid (saline + dextrose)
- Potassium chloride with calcium chloride and sodium chloride · Crystalloid IV fluid
- Potassium chloride with calcium chloride sodium chloride and sodium lactate · Balanced crystalloid IV fluid
- Potassium chloride with glucose · IV fluid (potassium + dextrose)
- Potassium chloride with sodium chloride · IV fluid (potassium + saline)
- Hyperkalaemia Management · UK Kidney Association Guidelines 2020; NICE CKD Guidelines
- Rhabdomyolysis · Renal Association 2018; UpToDate 2024
- Hypocalcaemia (Adult) · Society for Endocrinology
- SIADH (Endocrine Perspective) · European Hyponatraemia Guidelines 2014
- Hepatorenal Syndrome · EASL 2018; ICA 2015
- Acute Kidney Injury (AKI) · KDIGO 2012 / NICE AKI 2019
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.