MedCalcHub stores your favorites, history, and saved calculations locally in your browser using localStorage. This data never leaves your device.
Optional advertising, when enabled, may use third-party cookies or similar technologies. You can accept or reject advertising below.
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).
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
The serum anion gap is an acid-base screening calculation derived from routinely measured electrolytes: anion gap = Na - (Cl + HCO3). It approximates the concentration of unmeasured anions, notably albumin, phosphate, sulfate, and organic anions. In metabolic acidosis, a widened anion gap is classically associated with accumulation of unmeasured anions such as lactate, ketoacids, and ingested toxins. Albumin is a major contributor to the expected gap, so hypoalbuminemia lowers the expected anion gap and can mask a high anion gap metabolic acidosis. When albumin is low, an albumin-corrected calculation (adding roughly 2.5 mmol/L per 1 g/dL of albumin below 4.0 g/dL) is more appropriate. Because normal values vary with the laboratory method, interpret the result against the local laboratory reference range rather than a universal cutoff. The anion gap is a pattern-recognition tool within a broader acid-base assessment. It must be interpreted alongside the arterial blood gas, other electrolytes, the delta ratio, the osmolar gap, and the clinical context. A normal anion gap does not exclude metabolic acidosis, and an elevated gap does not by itself identify the underlying cause. This calculator performs a single arithmetic calculation for educational decision support. It does not diagnose acidosis or identify its mechanism; confirmatory laboratory assessment and clinical correlation are required.
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).
The anion gap is calculated as sodium minus the sum of chloride and bicarbonate (AG = Na - (Cl + HCO3)), all in mmol/L.
Serum sodium in mmol/L, used as the minuend.
Serum chloride in mmol/L, added to bicarbonate in the subtrahend.
Serum bicarbonate in mmol/L, added to chloride in the subtrahend.
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:
Expected Result: 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.