What is the formula for calculating the anion gap?

The correct formula for calculating the anion gap is derived from the need to assess the difference between measured cations and anions in serum. The anion gap is particularly useful in identifying the cause of metabolic acidosis.

The formula is calculated as the concentration of sodium (Na) minus the sum of chloride (Cl) and bicarbonate (HCO3) ions in the blood. By using this approach, the calculation effectively reflects the unmeasured anions in the serum, which can increase in various metabolic derangements, such as lactic acidosis or uremia.

In clinical practice, the standard reference range for the anion gap is typically around 8 to 12 mEq/L, though values can vary based on different laboratory standards. A significant elevation in the anion gap indicates the presence of additional unmeasured anions, assisting healthcare providers in diagnosing conditions that lead to metabolic acidosis.

Incorrect formulas, such as those that add the concentrations of chloride and bicarbonate or rearrange the components differently, do not accurately represent the ionic balance in the serum and thus would not yield a reliable anion gap for clinical assessment.