Anion Gap Calculator — Metabolic Acidosis Differential Diagnosis
Calculate the anion gap from serum sodium, chloride, and bicarbonate to differentiate anion-gap from non-anion-gap metabolic acidosis and guide diagnosis.
Anion Gap Calculator
Enter values below to calculate
Normal: 136–145 mEq/L.
Normal: 98–106 mEq/L.
Normal: 22–26 mEq/L. Use from metabolic panel or ABG.
Optional but recommended. Enter albumin for corrected AG. Normal: 3.5–5.0 g/dL.
This calculator is for informational and educational purposes. Results should be interpreted with clinical judgment and local guidelines.
What is the Anion Gap?
The Anion Gap (AG) is a calculated value representing the difference between measured cations (positively charged ions) and measured anions (negatively charged ions) in serum. In clinical practice, the formula AG = Na⁺ − (Cl⁻ + HCO₃⁻) is used. The "gap" reflects the unmeasured anions — primarily albumin, phosphate, sulfate, and organic acids — that are present in normal serum but not routinely measured. A normal anion gap is 8–12 mEq/L (with albumin of 4 g/dL) or 3–11 mEq/L by modern IDMS-calibrated assays. When metabolic acidosis is present, calculating the anion gap is the critical first step in differential diagnosis: a high anion gap (HAGMA) indicates accumulation of unmeasured organic acids (lactic acid, ketoacids, toxic alcohols), while a normal anion gap (NAGMA) indicates HCO₃⁻ loss or renal tubular acidosis. Albumin-corrected anion gap should be calculated whenever serum albumin is below 4 g/dL, as hypoalbuminemia lowers the normal anion gap by 2.5 mEq/L per 1 g/dL decrease.
Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻). Normal: 8–12 mEq/L (albumin 4 g/dL). Albumin-Corrected AG = Anion Gap + 2.5 × (4 − Albumin g/dL). AG > 12 mEq/L suggests high anion gap metabolic acidosis (HAGMA).
When to Use the Anion Gap Calculator
- Primary evaluation of any patient with metabolic acidosis (bicarbonate < 22 mEq/L or pH < 7.35 with low pCO₂) to classify as high anion gap (HAGMA) vs normal anion gap metabolic acidosis (NAGMA).
- ICU assessment of sepsis, diabetic ketoacidosis (DKA), lactic acidosis, renal failure, or ingestion of toxic alcohols (methanol, ethylene glycol) where elevated anion gap is a key diagnostic finding.
- Emergency department evaluation of altered consciousness, respiratory distress, unexplained tachycardia, or hypotension where metabolic acidosis may be contributing.
- Monitoring treatment response in DKA: Serial anion gap measurements confirm resolution of ketoacidosis (AG normalizing) even when glucose is still elevated.
- Evaluation of suspected renal tubular acidosis (RTA), Addison's disease, diarrhea-related HCO₃⁻ loss, or urinary diversion procedures causing NAGMA.
- Albumin-corrected anion gap calculation in hypoalbuminemic patients (cirrhosis, nephrotic syndrome, malnutrition) to avoid missing occult HAGMA.
Anion Gap Formula & Albumin Correction
Standard Anion Gap Formula
Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Na = 140, Cl = 104, HCO₃ = 24 → AG = 140 − (104 + 24) = 12 mEq/L (Normal)
Albumin-Corrected Anion Gap (Hypoalbuminemia)
Corrected AG = Measured AG + 2.5 × (4.0 − Albumin in g/dL)
AG = 14, Albumin = 2.0 g/dL → Corrected AG = 14 + 2.5 × (4−2) = 14 + 5 = 19 mEq/L (HAGMA!)
Delta-Delta Ratio (Concurrent NAGMA + HAGMA Assessment)
Delta-Delta = (AG − 12) / (24 − HCO₃). Ratio < 1 = concurrent NAGMA; 1–2 = pure HAGMA; > 2 = concurrent metabolic alkalosis
AG = 20, HCO₃ = 18 → ΔΔ = (20−12)/(24−18) = 8/6 = 1.33 → Pure HAGMA
Variables
Na⁺ (Serum Sodium) (mEq/L)
Primary measured cation. Normal: 136–145 mEq/L.
Cl⁻ (Serum Chloride) (mEq/L)
Primary measured anion. Normal: 98–106 mEq/L.
HCO₃⁻ (Serum Bicarbonate) (mEq/L)
Serum bicarbonate (from metabolic panel or ABG). Normal: 22–26 mEq/L.
Albumin (g/dL)
Required for albumin correction. Normal: 3.5–5.0 g/dL. Each 1 g/dL decrease below 4 reduces normal AG by 2.5 mEq/L.
Modern electrolyte analyzers using ion-specific electrodes may produce slightly lower chloride values, resulting in a lower calculated anion gap (3–11 mEq/L). Always use the reference range specific to your laboratory. The albumin-corrected anion gap should be calculated in ALL hypoalbuminemic patients to avoid missing occult HAGMA.
Anion Gap Interpretation — HAGMA vs NAGMA
The anion gap classifies metabolic acidosis into HAGMA (high anion gap) and NAGMA (normal anion gap), each with a distinct differential diagnosis.
| Range | Classification | Health Risk |
|---|---|---|
| < 3 mEq/L (Very low) | Very Low Anion Gap | Consider: hypoalbuminemia, multiple myeloma, severe hyperlipidemia. Recalculate albumin-corrected AG. |
| 3–11 mEq/L (Normal) | Normal Anion Gap | If acidosis present: NAGMA. Consider: RTA, diarrhea, saline infusion, ureteral diversion, Addison's disease. |
| 12–19 mEq/L (Mildly elevated) | Mildly Elevated — Borderline HAGMA | Evaluate for lactic acidosis, early DKA, uremia, or toxic ingestion. Check albumin-corrected AG. |
| 20–29 mEq/L (Elevated) | High Anion Gap Metabolic Acidosis | HAGMA confirmed. Use MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. |
| ≥ 30 mEq/L (Severely elevated) | Severely Elevated — Toxic/Critical | Severe HAGMA. Likely lactic acidosis (sepsis, shock), DKA, or toxic alcohol ingestion. Emergency assessment and ICU management. |
MUDPILES mnemonic for HAGMA causes: M = Methanol; U = Uremia; D = DKA; P = Propylene glycol (IV diazepam/lorazepam); I = Isoniazid/Iron/Inborn errors; L = Lactic acidosis; E = Ethylene glycol; S = Salicylates. DIARI mnemonic for NAGMA: D = Diarrhea; A = Addison's; I = IV saline; R = RTA; I = Ileal conduit/diversion.
Anion Gap Calculator Limitations & Clinical Warnings
Hypoalbuminemia effect: Each 1 g/dL decrease in serum albumin below 4 g/dL lowers the normal anion gap by approximately 2.5 mEq/L. Failure to correct for low albumin misses occult HAGMA in hypoalbuminemic patients (cirrhosis, malnutrition, nephrotic syndrome).
Laboratory reference range variation: Different analyzer methodologies produce different baseline AG reference ranges (8–12 mEq/L with older methods vs. 3–11 mEq/L with modern ion-specific electrode assays). Always use your laboratory's specific normal range.
Potassium exclusion: The formula Na − (Cl + HCO₃) excludes potassium. Some sources include potassium: AG = (Na + K) − (Cl + HCO₃), producing a normal range of approximately 12–16 mEq/L. Specify which formula your result uses.
Mixed acid-base disorders: The anion gap alone does not diagnose complex mixed disorders. Use the Delta-Delta ratio, pCO₂ analysis, and full ABG interpretation alongside electrolytes for complete acid-base assessment.
Hyperchloremia effect: Large volume normal saline infusion causes hyperchloremic (non-anion gap) acidosis that may mask a concurrent HAGMA. Albumin-corrected AG and lactate levels provide additional information.
Medical Disclaimer
Anion gap calculation is a component of comprehensive acid-base analysis and must be interpreted alongside arterial blood gas (ABG), clinical history, lactate, ketones, renal function, and toxicology screening. Management of metabolic acidosis requires urgent physician assessment.
References & Citations
- Emmett M, Narins RG. “Clinical use of the anion gap”. Medicine. 1977. 56(1): 38–54. PMID: 401921
- Figge J, Jabor A, Kazda A, Fencl V. “Anion gap and hypoalbuminemia”. Critical Care Medicine. 1998. 26(11): 1807–1810. PMID: 9824070 | DOI: 10.1097/00003246-199811000-00019
- Kraut JA, Madias NE. “Metabolic Acidosis: Pathophysiology, Diagnosis and Management”. Nature Reviews Nephrology. 2010. 6(5): 274–285. PMID: 20308999 | DOI: 10.1038/nrneph.2010.33
Medically Reviewed
Reviewed by CliniqWise Clinical Advisory Board — Nephrology & Critical Care Panel · Last updated: 2026-08-27 · Next review: 2027-02-27
Common Questions About Anion Gap Calculator
Explore More Medical Calculators
Browse the CliniqWise calculator library by specialty, score, disease area, or clinical task.
Browse All Calculators