What is the formula for the anion gap?

• A. Na - (Cl- + HCO3-)
• B. Na+ + Cl- - HCO3-
• C. Na+ - Cl- - HCO3-
• D. (Na+ - Cl- - HCO3-)

Answer: (A)

The anion gap is calculated to estimate unmeasured anions in the blood by comparing the main measured cations and anions. The standard approach uses sodium as the main measured cation and subtracts the sum of the main measured anions, chloride and bicarbonate. So the calculation is Na minus (Cl plus HCO3). This difference reflects unmeasured anions such as albumin, lactate, ketones, phosphate, and sulfate.

Normal values are roughly 8–12 mEq/L, and an increased gap points to accumulation of unmeasured anions in metabolic acidosis (for example, lactic acidosis or ketoacidosis). In contrast, some metabolic acidoses are non-gap (hyperchloremic), where bicarbonate falls but chloride rises to maintain electroneutrality, keeping the gap normal.

Some sources also show the formula including potassium (Na + K minus [Cl + HCO3]), but the simplified, commonly tested version omits potassium.