Anion Gap Calculator
Evaluate metabolic acidosis with optional albumin correction and delta ratio analysis.
Core Lab Tools provide rapid calculations and can be extended into full Clinical Intelligence Systems.
Input & Computation
In ICU patients, up to 50% of HAGMA cases are missed when albumin is not corrected for. A patient with albumin of 2 g/dL needs an AG correction of +5 mEq/L — this can be the difference between identifying a life-threatening HAGMA and missing it entirely.
Enter Electrolytes
All values in mEq/L unless noted. Albumin enables corrected AG.
Interpretation Guide
Formulae
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
AG(corr) = AG + 2.5 × (4 − Albumin g/dL)
Delta Ratio = (AG − 12) ÷ (24 − HCO₃)
Classification
- > 12 mEq/L — High AG metabolic acidosis
- 8–12 mEq/L — Normal AG
- < 8 mEq/L — Low AG — check albumin
Delta Ratio
- < 0.4— Normal AG / NAGMA
- 0.4–1.0— Mixed HAGMA + NAGMA
- 1.0–2.0— Pure HAGMA
- > 2.0— HAGMA + metabolic alkalosis
Evidence & References
Emmett M & Narins RG (1977)
Clinical use of the anion gap. Medicine 56(1):38–54. Original clinical validation of the anion gap concept.
Figge J et al. (1991)
Serum proteins and acid-base equilibria: a follow-up. Journal of Laboratory and Clinical Medicine 117(6):453–467. Basis for albumin correction formulas.
Feldman M et al. (2005)
Delta gap: an approach to complex acid-base disorders. Journal of the American Society of Nephrology 16(12):3744–3747.
Kraut JA & Madias NE (2007)
Serum anion gap: its uses and limitations in clinical medicine. Clinical Journal of the American Society of Nephrology 2(1):162–174.
CLSI EP28-A3c (2010)
Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory — reference range basis for ISE electrolyte measurements.
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). Modern ISE analysers yield reference range 8–12 mEq/L. Albumin correction formula: AG_corr = AG + 2.5 × (4 − Albumin g/dL). Delta ratio = (AG − 12) ÷ (24 − HCO₃) only valid when AG > 12.
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When to Use
- Metabolic acidosis workup — distinguish high vs normal anion gap causes
- ICU patients with unexplained acidaemia or multi-organ dysfunction
- Diabetic ketoacidosis monitoring — track treatment response
- Suspected toxic ingestion (salicylates, methanol, ethylene glycol)
- Renal disease with acid-base disturbance
- Characterising mixed acid-base disorders using delta ratio
Common Pitfalls
- Ignoring albumin — each 1 g/dL drop in albumin reduces AG by ~2.5 mEq/L, masking true HAGMA
- Lack of clinical correlation — always interpret AG alongside pH, pCO₂, and HCO₃
- Overreliance on AG alone — normal AG does not exclude metabolic acidosis
- Using older reference ranges (8–16 mEq/L) — modern ISE analysers give 8–12 mEq/L
- Ignoring delta ratio in DKA — AG resolves before HCO₃ recovers (hyperchloraemic phase)