Calculated Serum Osmolality
Calculates serum osmolality. Osmol gap = Measured − Calculated (>10 suggests toxic alcohol ingestion).
Score interpretation
Calculated osmolality 270–295 mOsm/kg: Normal range.
→ Compare to measured osmolality. Osmol gap (measured − calculated) > 10 mOsm/kg suggests toxic alcohols (methanol, ethanol, ethylene glycol, isopropanol).
Calculated osmolality < 270 mOsm/kg: Hypo-osmolar state.
→ Check measured osmolality. If low: confirms hypotonic hyponatraemia. Investigate: SIADH, hypothyroidism, adrenal insufficiency.
Calculated osmolality > 295 mOsm/kg: Hyperosmolar state.
→ If measured much higher than calculated (gap > 10): toxic alcohol ingestion, mannitol, or propylene glycol. DM/HHS: consider glucose contribution.
Interpretation bands for the Osmolality. Apply clinical judgement and local guidance.
References
- Bhagat CI et al. Calculated vs measured plasma osmolalities revisited. Clin Chem. 1984.
Related
Curated clinical cross-links plus same-class fallbacks.
- Chlorhexidine gluconate with isopropyl alcohol · Skin antiseptic
- Chlorhexidine · Antiseptic / disinfectant (bisbiguanide)
- Alcohol (ethanol) · Antidote / sclerosant
- Polyvinyl alcohol · Ocular lubricant
- Disulfiram · Alcohol Deterrent (Aldehyde Dehydrogenase Inhibitor)
- Acamprosate Calcium · Anti-craving Agent — Alcohol Relapse Prevention (GABA/Glutamate Modulator)
- Sepsis Screening and Sepsis Six · UK Sepsis Trust; NICE NG51; Surviving Sepsis Campaign 2021
- Unintentional Weight Loss Workup · NICE NG12; BSG
- Chronic Fatigue Workup · NICE NG206; BMJ Best Practice
- Lymphadenopathy Workup · NICE NG12; BMJ Best Practice
- Pre-op Medical Clearance · NICE NG45; ESC 2022
- Secondary Hypertension Workup · NICE NG136; ESH 2023
Featured in these MRCEM clinical pathways
The Osmolality is covered in detail — with RCEM/NICE evidence base, indications and pitfalls — in the following exam-focused pathways on our sister siteReviseMRCEM.
MRCEM Primary / Intermediate / OSCE candidates: each pathway includes exam-style questions, RCEM/NICE citations, and FAQ summaries.
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.