The difference between the concentration of measured cations and anions. The anion gap can be calculated by subtracting the concentration of routinely measured cations from routinely measured anions. Serum anion gap = [Na⁺] - ([Cl⁻] + [HCO₃⁻]). Normal range is 6–12 mEq/L. The anion gap can also be calculated taking potassium levels into account: ([Na⁺] + [K⁺]) - ([Cl⁻] + [HCO₃⁻]), with a normal range of 10–16 mEq/L.