Free Anion Gap Calculator
Optional
Including K⁺ changes the normal range to 5–16 mEq/L.
Enter Na⁺, Cl⁻, and HCO₃⁻ to calculate anion gap
The Free Anion Gap Calculator is an online medical tool designed to estimate the anion gap from serum electrolyte levels. By inputting sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻) concentrations, along with optional potassium (K⁺) and albumin values for correction, clinicians can quickly assess acid-base imbalances. This blood test calculator aids in identifying the underlying causes of metabolic acidosis.
Understanding the Anion Gap
In blood serum, the total positive charge from cations must balance the total negative charge from anions to maintain electrical neutrality. Standard laboratory panels measure only a subset of these ions; the anion gap represents the difference between the measured cations and the measured anions. It essentially quantifies the "unmeasured" ions (such as phosphate, sulfate, organic acids) that are not routinely assayed. Monitoring the anion gap is essential when evaluating patients with acidosis, as it helps differentiate between various pathological processes.
Anion Gap Formula
The classic calculation employs sodium as the primary cation and chloride plus bicarbonate as the measured anions:
All concentrations are expressed in milliequivalents per liter (mEq/L). If potassium is also measured, the formula can be expanded for greater precision:
Because potassium levels are relatively low, the simpler formula is often used in daily practice, but including potassium yields a more accurate result.
Normal Anion Gap and Interpretation
When potassium is omitted, the reference range is typically 3–11 mEq/L. With potassium included, the expected range rises to 5–16 mEq/L. The interpretation of results is as follows:
- High anion gap (>11 mEq/L without K⁺, >16 mEq/L with K⁺): Indicates a high anion gap metabolic acidosis, such as lactic acidosis or ketoacidosis.
- Normal anion gap (3–11 mEq/L without K⁺, 5–16 mEq/L with K⁺): Suggests a normal anion gap acidosis (hyperchloremic acidosis), where the primary defect is a loss of bicarbonate accompanied by a compensatory increase in chloride.
- Low anion gap (<3 mEq/L without K⁺, <5 mEq/L with K⁺): Often results from hypoalbuminemia, which lowers the unmeasured anion pool.
Albumin Correction
Low serum albumin (hypoalbuminemia) is frequent in critically ill patients and can obscure a mild elevation of the anion gap, leading to a false-normal reading. To overcome this, the anion gap should be corrected for albumin using the Figge–Jabor–Kazda–Fencl equation:
Where normal albumin is usually taken as 4.0 g/dL (or the local reference value). This adjustment "unmasks" any hidden anion gap elevation and allows for more accurate clinical interpretation. This tool provides educational reference and should not replace professional medical judgment.
FAQ
1. How is the anion gap calculated?
The standard formula is AG = [Na⁺] − ([Cl⁻] + [HCO₃⁻]). An expanded version including potassium is AG = [Na⁺] + [K⁺] − ([Cl⁻] + [HCO₃⁻]). All values are in mEq/L.
2. What is a normal anion gap range?
Without potassium, the normal range is 3–11 mEq/L. With potassium included, it is 5–16 mEq/L.
3. What does a high anion gap indicate?
A high anion gap (>11 mEq/L without K⁺, >16 mEq/L with K⁺) typically points to a high anion gap metabolic acidosis, such as lactic acidosis or ketoacidosis.
4. Why and how is the anion gap corrected for albumin?
Low albumin can mask an elevated anion gap. The correction uses the formula: Corrected AG = AG + 2.5 × (Normal Albumin − Measured Albumin), where normal albumin is usually 4.0 g/dL.
How to Use
- Enter the serum Sodium (Na⁺), Chloride (Cl⁻), and Bicarbonate (HCO₃⁻) levels in mEq/L.
- Optionally enter Potassium (K⁺) in mEq/L and Albumin in g/dL for corrected results.
- View the calculated anion gap, corrected anion gap, and interpretation of the results.