What is the Anion Gap?

The serum anion gap is a calculated clinical value used to identify the cause of metabolic acidosis. It represents the difference between the measured cations (positively charged ions) and the measured anions (negatively charged ions) in the blood serum. While the body maintains electrical neutrality, the "gap" exists because routine lab tests do not measure all ions, such as phosphates, sulfates, and organic acids.

How to Calculate the Anion Gap

The standard formula for calculating the anion gap (without potassium) is: Anion Gap = Sodium - (Chloride + Bicarbonate). When clinicians choose to include potassium, the formula becomes: (Sodium + Potassium) - (Chloride + Bicarbonate).

Interpreting the Results

A normal anion gap is typically between 8 and 12 mEq/L (if potassium is not included). If the gap is higher than normal, it indicates high-anion gap metabolic acidosis (HAGMA). This can be caused by various conditions often remembered by the mnemonic MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, and Salicylates.

Frequently Asked Questions

What causes a low anion gap?

A low anion gap is relatively rare but can be caused by hypoalbuminemia (low albumin levels), which reduces the unmeasured anions, or by an increase in unmeasured cations like lithium, calcium, or magnesium.

Does albumin affect the calculation?

Yes. Since albumin is the primary unmeasured anion, low levels of albumin will artificially lower the anion gap. For every 1 g/dL decrease in serum albumin below the normal range (4 g/dL), the observed anion gap should be corrected upward by approximately 2.5 mEq/L.