Anion Gap Calculator
Calculate the anion gap from electrolyte values to help assess acid-base balance.
What Is the Anion Gap?
The anion gap is a calculated value used in clinical medicine to help identify the cause of metabolic acidosis. It represents the difference between the primary measured cations (positively charged ions) and the primary measured anions (negatively charged ions) in the blood. A normal anion gap suggests certain types of acid-base disturbances, while an elevated or reduced gap points toward different underlying conditions.
This calculator uses standard serum electrolyte values — sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻) — to compute the gap. The result assists clinicians in narrowing the differential diagnosis for patients with acid-base imbalances.
How the Anion Gap Is Calculated
The most commonly used formula for the anion gap is:
Anion Gap = Sodium − (Chloride + Bicarbonate)
All values are expressed in milliequivalents per liter (mEq/L). The calculation assumes that the total number of positive charges in the blood equals the total number of negative charges. The gap exists because not all anions are routinely measured — unmeasured anions (such as albumin, phosphate, sulfate, and organic acids) account for the difference.
A normal reference range is typically 8 to 12 mEq/L, though this can vary slightly depending on the laboratory and the specific assay methods used. Some laboratories report a normal range of 7 to 13 mEq/L.
How to Use the Calculator
- Enter the patient's serum sodium level in mEq/L.
- Enter the serum chloride level in mEq/L.
- Enter the serum bicarbonate (CO₂ or HCO₃⁻) level in mEq/L.
- The calculator will automatically compute the anion gap.
Ensure all electrolyte values are from the same blood draw and measured in the same units. Mixing values from different time points or different units will produce an unreliable result.
Interpreting the Result
The calculated anion gap must be interpreted in the context of the patient's clinical presentation, history, and other laboratory findings. The result alone does not confirm a diagnosis.
Elevated Anion Gap
An anion gap above the normal reference range suggests the presence of unmeasured anions. Common causes include:
- Lactic acidosis (e.g., from sepsis, shock, hypoxia)
- Ketoacidosis (diabetic, alcoholic, or starvation)
- Renal failure (accumulation of uremic acids)
- Toxic ingestions (e.g., methanol, ethylene glycol, salicylates)
Normal Anion Gap
A normal anion gap in the presence of metabolic acidosis typically indicates a loss of bicarbonate or a gain of chloride. Common causes include:
- Diarrhea (bicarbonate loss)
- Renal tubular acidosis
- Acetazolamide use
- Hyperalimentation
Low Anion Gap
A low anion gap is less common but can occur with:
- Hypoalbuminemia (albumin is a major unmeasured anion)
- Multiple myeloma (due to cationic paraproteins)
- Lithium toxicity
- Laboratory error
Limitations and Considerations
The standard anion gap formula does not account for albumin, which is the most significant unmeasured anion. In patients with low albumin, the anion gap may be falsely normal. An albumin-corrected anion gap formula is sometimes used in critically ill patients or those with known hypoalbuminemia.
The reference range can vary between laboratories. Always use the reference range provided by the lab that processed the sample.
This calculator is intended for educational and clinical reference purposes. It does not replace professional medical judgment.
Frequently Asked Questions
What is a normal anion gap?
A normal anion gap is typically between 8 and 12 mEq/L, though some laboratories use a range of 7 to 13 mEq/L. The exact range depends on the lab's reference standards and measurement methods.
Why is albumin important for the anion gap?
Albumin carries a negative charge and is a major unmeasured anion. If albumin levels are low, the calculated anion gap will be lower than expected, potentially masking an elevated gap. An albumin-corrected formula adjusts for this.
Can the anion gap be negative?
A negative anion gap is rare but possible. It can occur with laboratory error, very high levels of unmeasured cations (such as in multiple myeloma), or severe hypercalcemia. A negative result should prompt a review of the electrolyte values and the patient's clinical status.
What is the difference between anion gap and osmolal gap?
The anion gap assesses acid-base balance using electrolytes. The osmolal gap compares measured and calculated serum osmolality and is used to detect the presence of unmeasured osmotically active substances like methanol or ethylene glycol. They are distinct calculations used for different clinical purposes.