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Anion Gap Calculator
The Anion Gap (AG) is a calculation used to identify the cause of metabolic acidosis. It represents the difference between the primary measured cations (Sodium) and the primary measured anions (Chloride and Bicarbonate) in serum.
This calculator uses the standard formula without Potassium, as Potassium fluctuates within a very narrow range and has minimal impact on the calculated gap in most clinical scenarios. A high anion gap typically indicates the presence of unmeasured anions (e.g., ketones, lactate).
Electrolytes
InputsUnits: mEq/L or mmol/L (values are identical).
Results
Normal Range: 8 – 12 mEq/L
(Some labs reference up to 16)
Clinical Context
Calculation Formula:
Anion Gap = [Na⁺] − ([Cl⁻] + [HCO₃⁻])
Interpretation:
• High Anion Gap (>12): Suggests the presence of unmeasured metabolic anions. Common causes are remembered by the mnemonic MUDPILES: Methanol, Uremia, DKA (Diabetic Ketoacidosis), Paraldehyde, Iron/Isoniazid, Lactic Acidosis, Ethylene Glycol, Salicylates.
• Normal Anion Gap: Suggests loss of bicarbonate or gain of chloride (Hyperchloremic Acidosis). Common causes include severe diarrhea or Renal Tubular Acidosis (RTA).
• Low Anion Gap (<8): Most commonly caused by Hypoalbuminemia (albumin is a major unmeasured anion). Other causes include Multiple Myeloma (cationic paraproteins) or Lithium toxicity.
Clinical References
How to Use This Calculator
Follow these steps to calculate the Anion Gap.
Input Electrolytes
Enter the serum Sodium (Na), Chloride (Cl), and Bicarbonate (HCO₃ or CO₂) values from the lab report.
Verify Units
Ensure all values are in mEq/L or mmol/L (these are interchangeable for monovalent ions).
Interpret Result
The calculated gap will appear. Compare this against the standard reference range (8-12).
Frequently Asked Questions
If you include Potassium in the formula: (Na + K) – (Cl + HCO3), the normal range is typically higher (approx. 12-16 mEq/L).
Yes. For every 1 g/dL decrease in albumin, the “normal” anion gap decreases by roughly 2.5 mEq/L. In hypoalbuminemia, the calculated gap must be corrected.