In the conventional anion gap calculation, which primary cation is used along with chloride and bicarbonate to estimate unmeasured anions?

• A. Sodium
• B. Potassium
• C. Calcium
• D. Magnesium

Answer: (A)

The idea behind the conventional anion gap is to estimate unmeasured anions by balancing the major measured cations and anions in the serum. The dominant extracellular cation is sodium, so the standard calculation uses sodium plus the major measured anions, chloride and bicarbonate, in the form Na+ minus (Cl− plus HCO3−). Sodium is used because it carries the bulk of the extracellular positive charge and remains relatively stable, making the difference a reliable proxy for unmeasured anions such as albumin, sulfate, phosphate, and organic acids. Potassium is sometimes added in alternative formulas, but the classic convention omits it to keep the gap simple and standardized. Calcium and magnesium are not used because they are present in much smaller amounts and their concentrations are more variable, which would distort the gap.