# Serum Anion Gap (AG) Calculator

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.

> Canonical page: https://elysiatools.com/en/tools/anion-gap-calculator

- **Category:** Health

- **Keywords:** anion gap, serum anion gap, AG, metabolic acidosis, HAGMA, NAGMA, hyperchloremic acidosis, lactic acidosis, ketoacidosis, albumin correction, electrolytes, nephrology

## Overview

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.

## Inputs

- **Include Potassium K⁺** (checkbox): Use AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻. Reference range shifts up to roughly 10–20 mmol/L.
- **Serum Sodium Na⁺ (mmol/L)** (number): Serum sodium (mmol/L). Physiologic range ~120–160.
- **Serum Chloride Cl⁻ (mmol/L)** (number): Serum chloride (mmol/L). Physiologic range ~80–130.
- **Serum Bicarbonate HCO₃⁻ (mmol/L)** (number): Serum bicarbonate / total CO₂ (mmol/L). Physiologic range ~5–50.
- **Serum Potassium K⁺ (mmol/L, optional)** (number): Serum potassium (mmol/L). Required only when 'Include Potassium K⁺' is checked.
- **Serum Albumin (g/dL, optional)** (number): Serum albumin (g/dL). If provided, computes albumin-corrected AG: AG_corr = AG + 2.5 × (4.0 − albumin).
- **Decimal Places** (number)

## When to use

- Assess whether metabolic acidosis has a high or normal anion gap.
- Check for a potentially masked high anion gap when serum albumin is low.
- Compare anion gap calculations using the standard formula or a formula that includes potassium.

## How it works

- Enter serum sodium, chloride, and bicarbonate values in mmol/L.
- Optionally enable potassium and enter a potassium value to use AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻.
- Optionally enter albumin in g/dL to calculate the albumin-corrected anion gap.
- The calculator returns the AG, optional corrected AG, formula used, and an anion-gap category.

## Use cases

- Screening electrolyte results for high anion gap metabolic acidosis.
- Reviewing possible hyperchloremic or normal-anion-gap metabolic acidosis.
- Adjusting the anion gap for hypoalbuminemia during clinical or educational review.

## Frequently asked questions

### What formula does the calculator use?

By default, it uses AG = Na⁺ − Cl⁻ − HCO₃⁻. If potassium is enabled, it uses AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻.

### What values are required?

Serum sodium, chloride, and bicarbonate are required. Potassium and albumin are optional.

### Why enter serum albumin?

Albumin correction can reveal a higher anion gap when hypoalbuminemia makes the uncorrected AG appear normal.

### How are high and normal anion gaps classified?

Without potassium, an AG above approximately 12 is generally high, while 8–12 is commonly considered normal. Reference ranges shift when potassium is included.

### Is this calculator medical advice?

No. Use the result as a calculation aid and interpret it with blood gases, laboratory data, and clinical findings.

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