What is the standard formula for the anion gap and a normal range?

• A. Na – (Cl + HCO3); normal range 8–12 mEq/L
• B. Na – Cl – HCO3; normal range 8–12 mEq/L
• C. Na + (Cl + HCO3); normal range 8–12 mEq/L
• D. Na – (Cl + HCO3); normal range 4–8 mEq/L

Answer: (A)

The key idea is that the anion gap estimates unmeasured anions in the blood. It’s calculated by taking the main positively charged ion measured in serum, sodium, and subtracting the main negatively charged ions, chloride and bicarbonate: AG = Na − (Cl + HCO3). In practice, many labs omit potassium, so the normal range for this non‑K formula is about 8–12 mEq/L. If potassium is included, the normal range shifts slightly upward, often around 12–16 mEq/L depending on the lab.

The expression Na − (Cl + HCO3) clearly shows subtracting the sum of the two major measured anions from sodium, which is the standard way to represent the calculation. Writing it as Na − Cl − HCO3 is mathematically equivalent, but the parentheses emphasize that you’re subtracting the combined amount of Cl and HCO3. Using addition, Na + (Cl + HCO3), would not reflect the concept and would yield a much larger value, not the anion gap. A normal range of 4–8 mEq/L is not the typical reference interval for this calculation.