Anion Gap

\[AG=Na-(Cl+HCO_3)\]

Variables

AGanion gap (mEq/L)

Na[Na⁺]serum sodium concentration (mEq/L)

Cl[Cl⁻]serum chloride concentration (mEq/L)

HCO3[HCO₃⁻]serum bicarbonate concentration (mEq/L)

Description

What is this formula?

The Anion Gap (AG) estimates the concentration of unmeasured ions in plasma. It is calculated from routinely measured electrolytes and is widely used in the evaluation of metabolic acidosis.

When to use it

Use this formula when evaluating acid-base disorders, metabolic acidosis, kidney disease, diabetic ketoacidosis, lactic acidosis, or toxic ingestions.

Example

Data:

Na = 140 mEq/L

Cl = 104 mEq/L

HCO3 = 24 mEq/L

Formula:

AG = Na − (Cl + HCO3)

Substitution:

AG = 140 − (104 + 24)

AG = 140 − 128

Result:

AG = 12 mEq/L

The calculated anion gap is 12 mEq/L.

Applications

- Acid-base disorder evaluation

- Metabolic acidosis classification

- Nephrology

- Emergency medicine

- Critical care medicine

Note

This is the classical anion gap formula that excludes potassium and remains the most widely taught and used version in clinical practice. Reference ranges vary among laboratories, and interpretation should consider serum albumin concentration because hypoalbuminemia can significantly reduce the measured anion gap. Albumin-corrected anion gap formulas are also commonly used.