A standard calculation from routine lab values
Anion gap is not measured directly — it's calculated from three values that typically already appear together on a basic metabolic panel: sodium, chloride, and bicarbonate.
Anion gap = Na − (Cl + HCO3)
The calculation reflects unmeasured anions in the blood (since labs don't routinely measure every ion present) and is used clinically as a starting point for narrowing down possible causes of certain acid-base disturbances.
A reference calculation, not a diagnosis
A normal anion gap is generally considered to fall roughly between 8 and 16 mEq/L, though reference ranges can vary slightly between labs and measurement methods. Values outside this range warrant clinical interpretation in context — by itself, this number narrows a differential rather than pointing to a single specific cause.