The anion gap
= (Na+ + K+) - (Cl- + HCO3-)
related to albumin
normal = 12 mEq/L (without potassium)
used to determine cause of metabolic acidosis
accounting for albumin
- the lower the albumin, the lower the upper limit
limitations
- high anion gap without acidosis can be caused by high albumin (paraprotein; gammaglobulinaemia)
- negative anion gap
- ↑ unmeasured cations (lithium, calcium, magnesium...)
- any halide interfering with chloride measurement