Calculator
For educational purposes only. Not a substitute for clinical judgment.
How to Calculate the Anion Gap
The serum anion gap equals sodium minus the sum of chloride and bicarbonate: AG = Na − (Cl + HCO3). Potassium is conventionally omitted because it is small and tightly regulated. The result represents unmeasured anions — mostly albumin, phosphate, sulfate, and organic acids. A normal anion gap is approximately 8–12 mEq/L, though the reference range varies by lab and by whether the analyzer reports lower chloride values.
Why Correct for Albumin
Albumin is the dominant unmeasured anion, contributing roughly 2.5 mEq/L to the gap for every 1 g/dL. In critically ill or malnourished patients with low albumin, a "normal" measured gap can hide a significant high-gap acidosis. The corrected anion gap adds 2.5 mEq/L for each 1 g/dL that albumin falls below 4.0 g/dL: Corrected AG = AG + 2.5 × (4.0 − albumin). Always interpret the corrected value when albumin is low.
High Anion Gap Differential: MUDPILES / GOLD MARK
A high anion gap metabolic acidosis reflects added unmeasured acid. The classic mnemonic MUDPILES stands for Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates. The updated GOLD MARK mnemonic (Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis) reflects modern toxicology. A normal-gap (hyperchloremic) acidosis instead points to bicarbonate loss — diarrhea or renal tubular acidosis.
Frequently Asked Questions
What is a normal anion gap?
A normal serum anion gap is approximately 8–12 mEq/L when calculated as Na − (Cl + HCO3). Some modern analyzers report chloride slightly higher, lowering the normal range to about 4–12 mEq/L, so always compare against your own lab reference range.
Why is potassium not included in the anion gap?
Potassium is usually omitted because its serum concentration is small and tightly regulated, so it contributes little to the calculation and its exclusion simplifies the reference range. Some formulas do include potassium, which raises the normal range by about 4 mEq/L.
How do you correct the anion gap for albumin?
Add 2.5 mEq/L to the measured gap for every 1 g/dL that serum albumin is below 4.0 g/dL: Corrected AG = measured AG + 2.5 × (4.0 − albumin). This matters because low albumin can mask a true high-gap acidosis in critically ill patients.
What does a high anion gap mean?
A high anion gap indicates unmeasured acid anions in the blood, most often from a high anion gap metabolic acidosis. Causes are captured by MUDPILES or GOLD MARK — including lactic acidosis, ketoacidosis, renal failure, and toxic ingestions such as methanol, ethylene glycol, and salicylates.
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