Anion Gap Explained Step by Step
Learn how to calculate the anion gap using Na − (Cl + HCO₃), understand normal vs high anion gap metabolic acidosis, and apply this clinically for ABG interpretation.
Anion Gap Formula
Core Formula
Na⁺ − (Cl⁻ + HCO₃⁻)
The anion gap helps identify unmeasured acids in metabolic acidosis and is a core concept in acid-base interpretation.
How to Calculate Anion Gap Step by Step
1. Check Sodium (Na⁺)
Start with the sodium value from the BMP or chemistry panel.
2. Add Chloride + Bicarbonate
Add the chloride (Cl⁻) and bicarbonate (HCO₃⁻) values together.
3. Subtract to Get Anion Gap
Subtract the sum from sodium to calculate the anion gap.
Normal vs High Anion Gap
Normal anion gap: ~8–12 mEq/L (lab dependent)
High anion gap: indicates unmeasured acids such as lactate, ketones, or toxins
Why the Anion Gap Matters
The anion gap is essential for evaluating metabolic acidosis. It helps differentiate between high anion gap acidosis (like DKA, lactic acidosis, renal failure) and normal anion gap acidosis (such as diarrhea or renal tubular acidosis).
Use the ABG Tool
Use the PulmTools ABG calculator to interpret acid-base disorders, calculate compensation, and apply the anion gap clinically.