Anion Gap and Osmolal Gap Calculator
Calculate the serum anion gap and the osmolal gap from routine chemistry results.
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For learning and checking your own calculations. Always follow your laboratory's SOPs and verified procedures.
Good to know
Anion gap = Na − (Cl + HCO₃), or (Na + K) − (Cl + HCO₃). Reference intervals depend on the analyser and method (often about 8–12 mmol/L without K, but some modern analysers give lower values), so use your laboratory's own range. Each 1 g/dL fall in albumin lowers the anion gap by about 2.5 mmol/L, so a 'normal' gap can hide a raised one when albumin is low. Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (mg/dL), or 2 × Na + glucose + urea (mmol/L). An osmolal gap above about 10 mOsm/kg suggests unmeasured osmoles such as ethanol, methanol or ethylene glycol. Interpret with the clinical picture and blood gases.
Formula
Anion gap = Na − (Cl + HCO₃), or (Na + K) − (Cl + HCO₃)
Albumin-corrected anion gap = anion gap + 2.5 × (4.0 − albumin g/dL)
Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (mg/dL), or 2 × Na + glucose + urea (mmol/L)
Osmolal gap = measured − calculated osmolality
Common questions
How is the anion gap calculated?
Na − (Cl + HCO₃). For example 140 − (104 + 24) = 12 mmol/L.
Why correct the anion gap for albumin?
Each 1 g/dL fall in albumin lowers the anion gap by about 2.5 mmol/L, which can hide a raised gap.
What does a high osmolal gap mean?
A gap above about 10 mOsm/kg suggests unmeasured osmoles such as ethanol, methanol or ethylene glycol.
Sources
- Kraut JA, Madias NE — Serum Anion Gap: Its Uses and Limitations in Clinical Medicine, Clin J Am Soc Nephrol 2007
- Tietz Fundamentals of Clinical Chemistry and Molecular Diagnostics (solutions, osmolality, clearance)