Anion Gap (AG)
The anion gap is a clinical calculation used to help identify the cause of metabolic
acidosis. It estimates the amount of unmeasured anions in plasma.
1. What Is the Anion Gap?
Plasma must remain electrically neutral:
Total positive charges (cations) = Total negative charges (anions)
But in routine lab tests, we measure only a few major ions.
Measured ions:
Cations
● Na⁺ (main one measured)
● sometimes K⁺
Anions
● Cl⁻
● HCO₃⁻
However, there are many unmeasured ions (proteins, phosphates, organic acids).
The anion gap estimates these unmeasured anions.
, 2. Formula for Anion Gap
Most commonly used formula:
AG = Na+ - (Cl- + HCO3-)
Sometimes potassium is included:
AG = (Na+ + K+) - (Cl- + HCO3-)
But many clinicians ignore K⁺ because it is small.
3. Normal Anion Gap
Normal range:
● 8–12 mEq/L (without K⁺)
● 12–16 mEq/L (with K⁺)
The main unmeasured anion contributing to AG is Albumin.
Other contributors:
● phosphate
● sulfate
● organic acids
The anion gap is a clinical calculation used to help identify the cause of metabolic
acidosis. It estimates the amount of unmeasured anions in plasma.
1. What Is the Anion Gap?
Plasma must remain electrically neutral:
Total positive charges (cations) = Total negative charges (anions)
But in routine lab tests, we measure only a few major ions.
Measured ions:
Cations
● Na⁺ (main one measured)
● sometimes K⁺
Anions
● Cl⁻
● HCO₃⁻
However, there are many unmeasured ions (proteins, phosphates, organic acids).
The anion gap estimates these unmeasured anions.
, 2. Formula for Anion Gap
Most commonly used formula:
AG = Na+ - (Cl- + HCO3-)
Sometimes potassium is included:
AG = (Na+ + K+) - (Cl- + HCO3-)
But many clinicians ignore K⁺ because it is small.
3. Normal Anion Gap
Normal range:
● 8–12 mEq/L (without K⁺)
● 12–16 mEq/L (with K⁺)
The main unmeasured anion contributing to AG is Albumin.
Other contributors:
● phosphate
● sulfate
● organic acids
