How is the anion gap calculated?

• A. Na − Cl − HCO3
• B. Na − HCO3 − Cl
• C. Na + Cl − HCO3
• D. Na − Cl + HCO3

Answer: (A)

The main idea is to estimate the amount of unmeasured anions by comparing the major measured cation to the major measured anions. The anion gap is calculated as the sodium concentration minus the sum of the chloride and bicarbonate concentrations: AG = Na+ − (Cl− + HCO3−). This is the standard way clinicians think about the gap, reflecting unmeasured anions that balance the charge.

In practice, this is often written as Na − Cl − HCO3, which is the same as Na − (Cl + HCO3). The normal range is about 8–12 mEq/L (without including potassium). A high gap suggests excess unmeasured acids (lactate, ketoacids, renal failure, toxins), while a normal gap metabolic acidosis points to bicarbonate loss with chloride rise (hyperchloremic). The form that subtracts the sum of chloride and bicarbonate from sodium best matches the established definition of the gap.