How the calculation works
The common calculation is Na − (Cl + HCO₃). Some laboratories use (Na + K) − (Cl + HCO₃), which yields a different number and reference range.
Worked example
With sodium 140, chloride 104 and bicarbonate 24 mEq/L, the usual potassium-excluded anion gap is 12 mEq/L.
Understanding the result
Anion gap reference ranges differ among laboratories and analytic methods. Albumin contributes to the measured gap; low serum albumin can mask an elevated gap.
Anion gap complements assessment of pH, bicarbonate, renal function, lactate, symptoms, and other clinical findings. It cannot identify an acid–base disorder alone.
Limitations
Do not apply a generic reference interval without checking the laboratory report and whether potassium is included.