The anion gap is calculated using which electrolytes?

• A. Na, Cl, HCO3
• B. Na, K, Cl
• C. Ca, Cl, HCO3
• D. K, Na, HCO3

Answer: (A)

An ion gap is a way to estimate unmeasured anions in the blood by comparing the main measured cation to the main measured anions. In routine serum chemistry, the main cation is sodium, and the major measured anions are chloride and bicarbonate. So the standard calculation is sodium minus the sum of chloride and bicarbonate. This gives the anion gap, which is typically about 8–12 mEq/L. Some formulas add potassium to both sides, giving [sodium plus potassium] minus [chloride plus bicarbonate], but the classic approach used in exams and practice uses only sodium, chloride, and bicarbonate. Calcium isn’t part of the calculation, and omitting either chloride or bicarbonate or replacing them with another ion makes the result no longer the standard anion gap.