How to Calculate Anion Gap
What is Anion Gap?
The anion gap detects unmeasured anions in plasma. AG > 12 mEq/L indicates high-anion-gap metabolic acidosis (DKA, lactic acidosis, toxins — MUDPILES mnemonic).
Formula
Anion gap = (Na) − (Cl + HCO3); Normal range 8–16 mEq/L; High AG suggests metabolic acidosis; Low AG rare, suggests lab error or hyperalbuminemia
- Na
- Serum sodium (mEq/L (135–145 normal))
- Cl
- Serum chloride (mEq/L (96–106 normal))
- HCO3
- Serum bicarbonate (mEq/L (22–26 normal))
Step-by-Step Guide
- 1AG = Na⁺ − (Cl⁻ + HCO₃⁻)
- 2Normal: 8–12 mEq/L
- 3Albumin correction: +2.5 mEq/L per 1g/dL drop in albumin below 4
- 4Investigate any AG > 12 mEq/L
Worked Examples
Input
Na 140, Cl 102, HCO₃ 14
Result
AG = 24 mEq/L — elevated, investigate
Frequently Asked Questions
Why is anion gap important?
Classifies causes of metabolic acidosis (high AG vs normal AG). High AG = organic acids (lactic acidosis, ketoacidosis). Normal AG = chloride-responsive (diarrhea, renal tubular acidosis).
What causes high anion gap acidosis?
Lactic acidosis (sepsis, hypoxia), ketoacidosis (diabetes, alcohol), renal failure, ingestions (salicylates, methanol, ethylene glycol). Mnemonic: MUDPILES.
What's the clinical significance?
High AG acidosis suggests serious underlying condition requiring urgent treatment. Normal AG suggests slower process (e.g., diarrhea). Treatment differs by cause.
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