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Calculate the serum Anion Gap (AG) to classify metabolic acidosis. Standard formula AG = Na⁺ − Cl⁻ − HCO₃⁻ (mmol/L), or AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻ with optional potassium. A high AG (≈ >12, or >20 with K⁺) indicates accumulation of unmeasured anions (lactic acidosis, ketoacidosis, renal failure, and toxins such as methanol/ethylene glycol, salicylates — mnemonic GOLD-MARK). A normal AG (8–12) during metabolic acidosis points to a hyperchloremic (normal-anion-gap) acidosis: diarrhea, renal tubular acidosis, saline resuscitation. A low AG (<3–6) is usually hypoalbuminemia, also hypercalcemia/hypermagnesemia, lithium, cationic IgG paraproteins, or bromide pseudo-hyperchloraemia. Optional albumin correction AG_corrected = AG + 2.5 × (4.0 − albumin g/dL) avoids missing a high-AG acidosis in hypoalbuminemia. Derived from Kraut CJASN 2007, Figge 1998, and MDCalc. Interpret with blood gas and full clinical context. Not medical advice.
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Tool usage guide
The Serum Anion Gap (AG) Calculator uses sodium, chloride, and bicarbonate values to calculate the anion gap and classify metabolic acidosis. You can optionally include potassium and albumin correction for hypoalbuminemia. Results are reported in mmol/L and should be interpreted with blood gas findings and the full clinical context.
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Background
A patient has sodium of 140 mmol/L, chloride of 104 mmol/L, and bicarbonate of 10 mmol/L.
Problem
Calculate the standard serum anion gap and classify the result.
How to use
Enter sodium 140, chloride 104, and bicarbonate 10. Leave potassium and albumin blank.
Outcome
The calculator returns AG = 26 mmol/L and classifies it as high anion gap metabolic acidosis.
Background
A patient has sodium of 138 mmol/L, chloride of 104 mmol/L, bicarbonate of 24 mmol/L, and albumin of 2.0 g/dL.
Problem
Determine whether hypoalbuminemia changes the interpretation of the anion gap.
How to use
Enter sodium 138, chloride 104, bicarbonate 24, and albumin 2.0. Do not enable potassium.
Outcome
The uncorrected AG is 10 mmol/L, while the albumin-corrected AG is 15 mmol/L.
By default, it uses AG = Na⁺ − Cl⁻ − HCO₃⁻. If potassium is enabled, it uses AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻.
Serum sodium, chloride, and bicarbonate are required. Potassium and albumin are optional.
Albumin correction can reveal a higher anion gap when hypoalbuminemia makes the uncorrected AG appear normal.
Without potassium, an AG above approximately 12 is generally high, while 8–12 is commonly considered normal. Reference ranges shift when potassium is included.
No. Use the result as a calculation aid and interpret it with blood gases, laboratory data, and clinical findings.