Anion Gap Calculator — AG = Na − (Cl + HCO₃)
The anion gap calculator computes AG = Na − (Cl + HCO₃) from serum electrolytes. Normal range is 8–12 mEq/L. An elevated anion gap indicates unmeasured anions (lactate, ketones, toxins). Optionally enter albumin for the Figge albumin-corrected AG, which uncovers hidden high-AG acidosis in hypoalbuminemic patients.
Medical Disclaimer
This tool is for informational and educational purposes only. Acid-base interpretation requires full clinical context. Consult a qualified healthcare professional for medical decisions.
Anion Gap Calculator
Calculate anion gap from serum electrolytes. Optionally enter albumin for albumin-corrected AG.
Normal: 136–145 mEq/L
Normal: 98–106 mEq/L
Normal: 22–29 mEq/L
Frequently Asked Questions
What is the anion gap formula?
Anion Gap = Sodium (Na⁺) − (Chloride (Cl⁻) + Bicarbonate (HCO₃⁻)). The normal range is 8–12 mEq/L. An elevated anion gap above 12 indicates the presence of unmeasured anions such as lactate, ketones, or toxins.
What does a high anion gap mean?
A high anion gap (> 12 mEq/L) indicates high anion gap metabolic acidosis. Common causes are remembered by the MUDPILES mnemonic: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates.
What does a normal anion gap mean?
A normal anion gap (8–12 mEq/L) metabolic acidosis is called 'hyperchloremic' or 'non-gap' metabolic acidosis. Common causes include diarrhea (bicarbonate loss), renal tubular acidosis (RTA), and administration of normal saline (dilutional acidosis). In the absence of acidosis, a normal anion gap is expected.
Why do we correct the anion gap for albumin?
Albumin is a negatively charged protein that contributes to the anion gap. When albumin is low (hypoalbuminemia), the anion gap is falsely low. The albumin-corrected AG (Figge formula: AG + 2.5 × (4 − albumin)) gives the true gap, which may be elevated even when the uncorrected AG appears normal.
What is a low anion gap?
A low anion gap (< 8 mEq/L) is uncommon and can be caused by hypoalbuminemia (the most common cause), multiple myeloma (positively charged paraproteins), lithium toxicity, or laboratory error (hyperviscosity). Always calculate the albumin-corrected anion gap when albumin is low.
What is the delta gap (delta-delta)?
The delta gap compares the rise in anion gap to the expected fall in bicarbonate. It helps detect mixed acid-base disorders: if the corrected AG is elevated but HCO₃ is not as low as expected, there may be a concurrent metabolic alkalosis. Delta gap = corrected AG − 12. A value > 12 suggests concurrent metabolic alkalosis.
What are normal values for sodium, chloride, and bicarbonate?
Normal serum values: Sodium (Na⁺): 136–145 mEq/L. Chloride (Cl⁻): 98–106 mEq/L. Bicarbonate (HCO₃⁻): 22–29 mEq/L. Albumin: 3.5–5.0 g/dL. Deviations from these ranges affect the anion gap calculation.
How is the anion gap used in clinical practice?
The anion gap is used as a screening tool when metabolic acidosis is detected on blood gas analysis. An elevated anion gap immediately triggers investigation for the specific cause. It is part of the systematic approach to acid-base analysis along with pH, pCO₂, HCO₃, and albumin-corrected values.
About the Anion Gap Calculator
About This Calculator
The anion gap (AG) is a derived value calculated from serum electrolytes that helps classify metabolic acidosis and identify its underlying cause. It represents the difference between measured cations (sodium) and measured anions (chloride + bicarbonate).
An elevated anion gap indicates the presence of unmeasured anions (e.g., lactate, ketones, toxins), which is the hallmark of high anion gap metabolic acidosis.
Anion Gap Formula
AG = Na&sup+ − (Cl&sup- + HCO&sub3;&sup-)
Normal range: 8–12 mEq/L
Example: Na = 140, Cl = 100, HCO₃ = 24 → AG = 140 − (100 + 24) = 16 mEq/L (elevated, high anion gap)
Albumin-Corrected Anion Gap (Figge Formula)
Albumin is a negatively charged protein that contributes to the anion gap. When albumin is low, the anion gap may be falsely normal even in the presence of unmeasured anions.
Corrected AG = AG + 2.5 × (4.0 − Albumin)
Always calculate the albumin-corrected AG in critically ill patients, who commonly have hypoalbuminemia that masks a high anion gap metabolic acidosis.
Interpretation
| AG Value | Interpretation | Consider |
|---|---|---|
| < 8 mEq/L | Low Anion Gap | Hypoalbuminemia, multiple myeloma, lithium toxicity |
| 8–12 mEq/L | Normal | Normal AG metabolic acidosis (diarrhea, RTA) or no acidosis |
| > 12 mEq/L | High Anion Gap | Unmeasured anions — use MUDPILES mnemonic |
High Anion Gap Causes — MUDPILES Mnemonic
Delta Gap (Delta-Delta)
The delta gap helps identify mixed acid-base disorders. It compares the rise in anion gap to the fall in bicarbonate:
Delta Gap = Corrected AG − 12
- Delta gap < 0: Suggests concurrent normal AG acidosis
- Delta gap 0–12: Pure high AG metabolic acidosis
- Delta gap > 12: Suggests concurrent metabolic alkalosis
Medical Disclaimer
This calculator is for educational and informational purposes only. Acid-base interpretation requires complete clinical context. Consult a qualified healthcare professional for diagnosis and treatment.