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Metabolic Calculator

Anion Gap Calculator

Calculate serum anion gap with albumin correction. Classify metabolic acidosis type.

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Enter serum values in mmol/L (or mEq/L — equivalent for these electrolytes).
⚠️ For clinical documentation only. Interpret in full clinical context.
Anion Gap Result
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Anion Gap: The Complete Clinical Guide

What the anion gap measures, how to calculate it, albumin correction, HAGMA vs NAGMA, delta-delta ratio, and clinical management

🧪 What Is It 📐 Formula 🔴 HAGMA 🟡 NAGMA

What Is the Anion Gap?

The anion gap (AG) is a calculated value reflecting the difference between measured cations and measured anions in serum. It represents unmeasured anions (albumin, phosphate, sulphate, organic acids) that are not routinely measured. In health, the AG is 8–12 mmol/L. An elevated AG indicates accumulation of pathological unmeasured anions — most commonly lactate, ketoacids, or toxins.

Formula & Albumin Correction

Standard Formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal range: 8–12 mmol/L
Albumin-Corrected AG (Figge Formula)
Corrected AG = AG + 2.5 × (4.0 − albumin g/dL)
Critical in hypoalbuminaemia — low albumin masks an elevated AG. Always correct when albumin <4.0 g/dL.

High Anion Gap Metabolic Acidosis (HAGMA)

HAGMA occurs when pathological unmeasured anions accumulate — most commonly lactate (lactic acidosis), ketoacids (DKA), uraemic acids, or exogenous toxins. The MUDPILES mnemonic covers the major causes. Once HAGMA is identified, calculate the delta-delta ratio to detect a mixed disorder.

Delta-Delta Ratio
ΔΔ = (AG − 12) ÷ (24 − HCO₃⁻)
<0.4 = Pure NAGMA · 0.4–0.8 = Mixed HAGMA + NAGMA · 1–2 = Pure HAGMA · >2 = HAGMA + metabolic alkalosis

Normal Anion Gap Metabolic Acidosis (NAGMA)

NAGMA results from direct bicarbonate loss (GI or renal) with compensatory chloride retention — hence also called hyperchloraemic metabolic acidosis. The most common causes are diarrhoea and renal tubular acidosis (RTA). Use urinary anion gap to differentiate renal from GI causes: negative UAG suggests GI loss; positive UAG suggests RTA.

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References

  1. Emmett M, Narins RG. Clinical use of the anion gap. Medicine. 1977;56(1):38-54.
  2. Figge J, Rossing TH, Fencl V. The role of serum proteins in acid-base equilibria. J Lab Clin Med. 1992;117(6):453-467.
  3. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162-174.
  4. BMJ Best Practice. Approach to metabolic acidosis. Updated 2024.

⚠️ Medical Disclaimer: This calculator is for educational and clinical documentation purposes only. Always interpret results in the context of the full clinical picture. MedDraftPro accepts no clinical liability.

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