Anion Gap Calculator (mmol/L)
Calculate serum anion gap, albumin-corrected AG and delta gap to help evaluate metabolic acidosis.
Last updated on: July 15, 2025
Serum Anion Gap & Albumin-Corrected AG
What is the anion gap?
The anion gap (AG) is a calculated value derived from routinely measured electrolytes (Na⁺, Cl⁻, HCO₃⁻ ± K⁺). It represents the difference between measured cations and measured anions in plasma, and serves as a surrogate for unmeasured anions.
In clinical practice, the anion gap is mainly used to evaluate metabolic acidosis and to detect the presence of unmeasured anions such as lactate, ketone bodies, uraemic toxins, or certain poisons.
Why use the anion gap calculator?
- Rapidly estimate whether a metabolic acidosis is high anion gap or normal anion gap.
- Identify possible contributions from unmeasured anions (e.g. lactate, ketones, toxins) in critically ill patients.
- Apply albumin correction and delta gap (ΔAG) to refine the assessment of complex or mixed acid–base disorders.
How to interpret the anion gap
Limitations & important notes
The anion gap is a helpful screening and classification tool, but it must always be interpreted alongside pH, PaCO₂, clinical history and other laboratory data.
- Reference ranges vary between laboratories depending on the analyser and electrolyte methods used.
- Hypoalbuminaemia lowers the anion gap; albumin-corrected AG may be more informative in critically ill or malnourished patients.
- A normal anion gap does not exclude clinically significant metabolic acidosis, particularly hyperchloraemic acidosis.
- Spurious results can occur with severe hyperlipidaemia, paraproteinaemia, or measurement error – consider repeating the test if the result is inconsistent with the clinical picture.
This page has been medically reviewed by Dr. Khoulah Attia – PharmD, Immunology Specialist.