Looking for more details? Read our comprehensive Anion Gap guide
Why Calculate the Anion Gap?
The anion gap helps determine the cause of metabolic acidosis by estimating the quantity of unmeasured anions in the blood, such as lactate, ketones, and toxic metabolites.
In a healthy individual, the sum of cations (positively charged ions) equals the sum of anions (negatively charged ions) to maintain electrical neutrality. However, routine lab tests do not measure all ions. The "gap" represents the unmeasured anions.
Formula
Interpretation (General Guidelines)
- Normal Range: ~ 8 - 12 mEq/L (may vary by lab assay)
- High Anion Gap: > 12 mEq/L (Suggests High Anion Gap Metabolic Acidosis)
- Low Anion Gap: < 8 mEq/L (Rare, often due to hypoalbuminemia or paraproteinemia)
Note: Normal reference ranges may vary slightly between laboratories depending on the equipment used. Always refer to your local laboratory's established values.
Limitations
The anion gap relies on measured ions and can be altered by various factors:
- Hypoalbuminemia: Albumin is a major unmeasured anion. Low albumin falsely lowers the anion gap. The gap should be corrected for albumin: add 2.5 mEq/L to the AG for every 1 g/dL decrease in albumin below 4.0 g/dL.
- Potassium: Some formulas include potassium (Na⁺ + K⁺) - (Cl⁻ + HCO₃⁻), which increases the normal range by ~4 mEq/L. This calculator uses the more common formula without potassium.
- Laboratory error: Errors in sodium, chloride, or bicarbonate measurement will skew the result.
How to Use
Enter the patient's serum Sodium (Na), Chloride (Cl), and Bicarbonate (HCO3). If Potassium (K) is relevant for your calculation, include it (optional). Enter Albumin to calculate the Albumin-corrected Anion Gap.
Case Example
Patient: 40-year-old female with suspected diabetic ketoacidosis. Na: 140, Cl: 100, HCO3: 15, Albumin: 4.0.
Calculation: Gap = Na - (Cl + HCO3) = 140 - (100 + 15) = 25 mEq/L.
Interpretation: High anion gap (normal is roughly 8-12). This points towards an anion gap metabolic acidosis (e.g., DKA, lactic acidosis).
Frequently Asked Questions
Albumin is a major unmeasured anion. In patients with hypoalbuminemia, the baseline anion gap is lower. Correcting it helps avoid missing an elevated gap acidosis in malnourished or critically ill patients.
Traditionally, potassium is omitted because its extracellular concentration is low and relatively stable. However, some formulas include it. If included, the normal range changes.