Anion Gap

Measured in mmol/L

A calculated balance check on the electrolytes in your blood — it doesn't measure anything new, it compares values already on your panel to flag possible acid-base imbalance.

What it is

The anion gap isn't measured — it's arithmetic done on three electrolytes your panel already reports, usually sodium minus chloride minus bicarbonate. Blood has to stay electrically balanced overall, and the ions labs routinely measure don't account for all of it; the leftover difference is the “gap”, standing in for the ions nobody counted. Because it's derived, it moves whenever any of its inputs move, and a change can reflect the arithmetic rather than anything new about you. Clinically it's used mainly as a first sorting step when bicarbonate is low, helping narrow why. On a routine panel in someone who feels well it's rarely the interesting number, and it's read alongside the electrolytes it came from rather than on its own.

Worth asking your clinician

Which formula does my lab use for the anion gap, and what range applies to it?