Anion Gap Formula
Calculate the anion gap: AG = Na+ - (Cl- + HCO3-).
Used to diagnose metabolic acidosis and identify its underlying cause.
The Formula
The anion gap is a calculated value used in clinical medicine to help diagnose the cause of metabolic acidosis. It represents the difference between the measured cations (positively charged ions) and the measured anions (negatively charged ions) in the blood. The "gap" exists because not all ions are routinely measured — unmeasured anions like lactate, ketoacids, and other organic acids fill the gap.
The normal anion gap is typically 8 to 12 mEq/L, though this varies slightly between laboratories depending on the analyzers used. An elevated anion gap (above 12 mEq/L) suggests that excess unmeasured anions are present in the blood, which points to specific causes of metabolic acidosis.
Clinicians use the mnemonic MUDPILES to remember the common causes of an elevated anion gap: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid or Iron, Lactic acidosis, Ethylene glycol, and Salicylates. Each of these conditions produces acids that increase the unmeasured anion load in the blood.
A normal anion gap with metabolic acidosis (called non-anion gap metabolic acidosis) suggests a different set of causes, usually related to bicarbonate loss through the kidneys or gastrointestinal tract. Common causes include diarrhea, renal tubular acidosis, and certain medications.
The anion gap is one of the first calculations performed when a patient presents with an abnormal blood gas or low bicarbonate level. It is quick, inexpensive, and provides immediate diagnostic direction.
Your data stays in your browser. We do not store, collect, or transmit any information you enter.
Variables
| Symbol | Meaning |
|---|---|
| AG | Anion gap (mEq/L) |
| Na+ | Serum sodium concentration (mEq/L) |
| Cl− | Serum chloride concentration (mEq/L) |
| HCO3− | Serum bicarbonate concentration (mEq/L) |
Example 1
Problem
A patient has Na+ = 140 mEq/L, Cl− = 104 mEq/L, and HCO3− = 24 mEq/L. Calculate the anion gap.
AG = 140 − (104 + 24)
AG = 140 − 128
AG = 12 mEq/L. This is within the normal range (8-12 mEq/L), indicating no significant unmeasured anion excess.
Example 2
Problem
A diabetic patient presents with Na+ = 138 mEq/L, Cl− = 98 mEq/L, and HCO3− = 14 mEq/L. Is the anion gap elevated?
AG = 138 − (98 + 14)
AG = 138 − 112
AG = 26 mEq/L. This is significantly elevated above normal, strongly suggesting an anion gap metabolic acidosis — likely diabetic ketoacidosis given the clinical context.
When to Use It
The anion gap calculation is a fundamental tool in emergency and internal medicine for evaluating acid-base disorders.
- Diagnosing the cause of metabolic acidosis when bicarbonate is low
- Differentiating between anion gap and non-anion gap metabolic acidosis
- Monitoring treatment response in diabetic ketoacidosis or lactic acidosis
- Screening for toxic ingestions (methanol, ethylene glycol, salicylates)