The anion gap (AG) is a simple calculated parameter from serum electrolytes that helps systematically narrow the cause of metabolic acidosis. It is one of the most important tools in emergency and critical care medicine.
This guide explains how to calculate the anion gap, what the normal values mean, when albumin correction is necessary, and which diseases can cause an elevated anion gap.
The Formula
The anion gap is the difference between the measured cation sodium and the measured anions chloride and bicarbonate:
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
The reference range is 8–12 mEq/L (without potassium). The "gap" exists because not all anions in serum are routinely measured — albumin, phosphate, and others contribute to charge neutrality.
| Anion Gap | Interpretation |
|---|---|
| < 8 mEq/L | Low |
| 8 – 12 mEq/L | Normal |
| 13 – 20 mEq/L | Mildly elevated |
| > 20 mEq/L | High — High-AG acidosis |
Why Albumin Correction Matters
Albumin is the most important "unmeasured anion" in serum. In hypoalbuminemia (e.g., liver disease, malnutrition, critically ill patients) the anion gap is falsely low — a high-AG acidosis can be missed.
Corrected AG = AG + 2.5 × (4.0 − Albumin [g/dL])
A patient with albumin 2.0 g/dL and AG 10 mEq/L has a corrected AG of 10 + 2.5 × (4.0 − 2.0) = 15 mEq/L — a mildly elevated gap despite an apparently normal raw value.
High-AG Acidosis: MUDPILES
An anion gap above 20 mEq/L suggests high-AG metabolic acidosis. The mnemonic MUDPILES helps recall the most important causes:
| Letter | Cause |
|---|---|
| M | Methanol |
| U | Uremia |
| D | Diabetic Ketoacidosis (DKA) |
| P | Propylene glycol |
| I | Iron / Isoniazid |
| L | Lactic acidosis |
| E | Ethylene glycol |
| S | Salicylates (aspirin) |
Low Anion Gap
An AG below 8 mEq/L is less common but clinically relevant. Common causes include:
- Hypoalbuminemia — most common cause of a low anion gap
- Hyperchloremia — e.g., from saline infusion or hyperchloremic acidosis
- Bromide intoxication — bromide is measured as chloride
- Lithium therapy — lithium is a cation that lowers the AG
Related Lab Parameters
The anion gap is part of a broader metabolic workup. Related parameters:
- GFR (Glomerular Filtration Rate) — kidney function is key for uremia and renal acidosis. Calculate your GFR value.
- HbA1c — long-term blood glucose for assessing diabetic decompensation. To the HbA1c Converter.
- Blood Sugar — glucose is central to diagnosing DKA. To the Blood Sugar Converter.
Calculate Your Anion Gap Now
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Open Anion Gap Calculator →Frequently Asked Questions
Why is potassium omitted from the anion gap formula?
The classic formula AG = Na − (Cl + HCO₃) omits potassium because its concentration is much lower than sodium. Some formulas add potassium to sodium, which shifts the normal range slightly higher (to 10–14 mEq/L). Clinical practice typically uses the formula without potassium.
Can I calculate the anion gap without a blood gas?
Yes. Sodium and chloride come from the serum electrolyte panel; bicarbonate (HCO₃⁻) from a blood gas or as serum bicarbonate from a routine metabolic panel. Values from different draw times should not be mixed.
Does the normal range apply to all patients?
The normal range of 8–12 mEq/L assumes normal albumin. In hypoalbuminemia the albumin-corrected anion gap must always be calculated, otherwise a high-AG acidosis can be missed.