Anion Gap Calculator

All three inputs carry the same weight of 1 in the formula (AG = Na+ - Cl- - HCO3-), but sodium's normal reference value is roughly 30-40% higher than chloride's and several times higher than bicarbonate's, so an equivalent percentage swing in sodium represents a larger absolute mEq/L change and moves the calculated gap the most in practice, even though no single term is mathematically weighted more than another.

The plain anion gap formula (Na+ minus chloride plus bicarbonate) has no albumin term at all -- albumin only enters through the separate correction, which adds roughly 2.5 mEq/L for every 1 g/dL the measured albumin sits below the normal reference of 4.0 g/dL. That correction exists because albumin is the largest unmeasured anion in serum, so a low albumin from malnutrition or critical illness can mask a genuinely elevated gap if only the uncorrected number is read.

A gap above the roughly 8-12 mEq/L normal range means some acid has accumulated in the blood that isn't being captured by the routine chloride and bicarbonate measurements -- common causes include diabetic ketoacidosis, lactic acidosis, uremia from kidney failure, and certain toxic ingestions like methanol or ethylene glycol. A normal gap with acidosis instead points toward bicarbonate being lost directly, such as from severe diarrhea or a renal tubular disorder.

It compares the rise in the anion gap above its own normal value to the fall in bicarbonate below its own normal value. A ratio near 1 is consistent with a single, uncomplicated high-gap acidosis, while a ratio noticeably higher or lower than 1 suggests a second, coexisting acid-base disturbance -- such as a concurrent metabolic alkalosis or a mixed high- and normal-gap acidosis -- that the anion gap alone would not reveal.