Anion Gap Calculator
Calculate the anion gap from serum sodium, chloride, and bicarbonate.
Returns AG value, clinical category (normal, elevated, high, critical), and MUDPILES causes.
The anion gap is a calculated value used in clinical medicine to evaluate metabolic acidosis. It is the difference between the routinely measured cations (positive ions) and anions (negative ions) in the blood. The “gap” exists because not every ion is measured. Unmeasured anions like lactate, ketoacids, and other organic acids fill the gap, and an elevated gap points directly to specific causes.
The formula: AG = Na⁺ − (Cl⁻ + HCO₃⁻)
All three inputs are serum concentrations in mEq/L.
Reference ranges:
| Anion Gap (mEq/L) | Interpretation |
|---|---|
| 8 to 12 | Normal range |
| 13 to 17 | Mildly elevated, investigate cause |
| 18 to 25 | Significantly elevated, likely metabolic acidosis |
| Above 25 | Critical, urgent workup required |
The “normal” range varies slightly between laboratories (some report 6 to 12 or 10 to 14) depending on the analyzer and the inclusion of potassium. The values above assume the standard three-electrolyte form without K⁺.
MUDPILES: the elevated anion gap mnemonic
Clinicians remember the common causes of an elevated anion gap with MUDPILES:
- M ethanol
- U remia (kidney failure)
- D iabetic ketoacidosis
- P ropylene glycol
- I soniazid or Iron
- L actic acidosis
- E thylene glycol
- S alicylates (aspirin toxicity)
Each of these conditions produces acids that increase the unmeasured anion load.
Normal anion gap metabolic acidosis
A low or normal AG with metabolic acidosis (low HCO₃⁻) is a different problem entirely. The cause is usually bicarbonate loss through the kidneys or gut. Common causes: chronic diarrhea, renal tubular acidosis, post-resuscitation with normal saline, and acetazolamide therapy.
Worked example: healthy patient
Na⁺ = 140, Cl⁻ = 104, HCO₃⁻ = 24 AG = 140 − (104 + 24) = 140 − 128 = 12 mEq/L, top of the normal range.
Worked example: diabetic ketoacidosis
Na⁺ = 138, Cl⁻ = 98, HCO₃⁻ = 14 AG = 138 − (98 + 14) = 138 − 112 = 26 mEq/L, significantly elevated and fits DKA in the right clinical context.
The corrected anion gap
In patients with abnormal serum albumin, the AG should be corrected because albumin is itself a major unmeasured anion. The Figge correction adds 2.5 mEq/L per 1.0 g/dL drop in albumin below 4.0 g/dL. A hypoalbuminemic patient with an “apparently normal” AG of 10 may actually have an elevated corrected AG of 14 or higher. This is the most common pitfall in interpreting the gap in hospitalized patients.
When to use it
- Diagnosing the cause of metabolic acidosis when serum bicarbonate is low
- Differentiating anion gap from non-anion gap metabolic acidosis
- Monitoring treatment response in diabetic ketoacidosis or lactic acidosis
- Screening for toxic ingestions (methanol, ethylene glycol, salicylates) when clinical suspicion exists
Your data stays in your browser. We do not store, collect, or transmit any information you enter.