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Calculates the serum anion gap using sodium, chloride, and bicarbonate. The anion gap helps differentiate high anion gap metabolic acidosis (HAGMA) from normal anion gap metabolic acidosis (NAGMA).
Anion Gap = Na − (Cl + HCO₃)
8–12 mmol/L
Interpret results together with the patient's clinical presentation.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-06
Formula
Keywords
Normal Range
8–12 mmol/L
| Classification | Range |
|---|---|
Low anion gap | <7.1 |
Normal anion gap | 8–12 |
High anion gap | ≥13 |
Markedly elevated anion gap | ≥20 |
Calculates the serum anion gap to help differentiate high anion gap metabolic acidosis (HAGMA) from normal anion gap metabolic acidosis (NAGMA).
An anion gap of 8–12 mmol/L is normal. Values above 12 mmol/L in the context of metabolic acidosis suggest HAGMA. Values below 7 mmol/L may indicate hypoalbuminemia or laboratory error.
The anion gap is a fundamental tool in acid-base interpretation. An elevated anion gap in the setting of metabolic acidosis indicates accumulation of unmeasured anions and requires urgent investigation of the underlying cause.
A 54-year-old man with known diabetes presents with Kussmaul breathing. Sodium is 140 mmol/L, chloride is 105 mmol/L, bicarbonate is 12 mmol/L.
Inputs:
Anion gap = 140 − (105 + 12) = 23 mmol/L. This is markedly elevated and suggests a high anion gap metabolic acidosis, likely diabetic ketoacidosis.
An elevated anion gap (> 12 mmol/L) in the setting of metabolic acidosis suggests accumulation of unmeasured anions such as lactate, ketoacids, or toxic metabolites. Common causes include lactic acidosis, diabetic ketoacidosis, renal failure, and toxic alcohol ingestion.
A low anion gap (< 8 mmol/L) may indicate hypoalbuminemia, lithium or bromide toxicity, or laboratory error. It can also be seen with hypercalcemia, hypermagnesemia, or hyperkalemia.
About 80% of the normal anion gap is accounted for by albumin. In hypoalbuminemia, the anion gap is falsely low, potentially masking a high anion gap metabolic acidosis. For every 1 g/dL drop in albumin below 4.0, the expected anion gap decreases by approximately 2.5 mmol/L.
Use the albumin-corrected anion gap when the patient has known or suspected hypoalbuminemia (e.g. critical illness, nephrotic syndrome, liver disease, malnutrition).
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. It does not replace clinical judgment. Always interpret laboratory values in the context of the individual patient's clinical presentation.
Screening for high anion gap metabolic acidosis.
Does not account for hypoalbuminemia.
Detecting hidden HAGMA in hypoalbuminemic patients.
Requires albumin measurement.
Assessing osmolality in toxic ingestions and electrolyte disorders.
Does not directly measure the anion gap.