Corrected Anion Gap Formula (Figge Equation)
The corrected anion gap formula — AG + 2.5 × (4.0 − albumin) — restores a hidden high-anion-gap metabolic acidosis that hypoalbuminemia masks. Here is the derivation, why the factor is 2.5, and worked examples.
The Figge equation
The corrected anion gap formula, also called the Figge equation, adjusts the observed anion gap for the patient's serum albumin. Albumin is the dominant "unmeasured" anion in plasma — it is polyvalent and negatively charged at physiological pH, so it contributes the bulk of the gap under normal conditions. When albumin falls, the entire anion gap falls with it, and a patient with a genuine high-anion-gap metabolic acidosis can show a falsely reassuring "normal" gap. The correction restores the gap to what it would read if albumin were normal at 4.0 g/dL:
The equation is the practical answer to the question why albumin matters when you read an anion gap. Each 1 g/dL fall in serum albumin lowers the expected "normal" gap by roughly 2.5 mEq/L, so the correction adds back that hidden 2.5 mEq/L per gram of deficit — and subtracts it when albumin is high. The result is a gap that can be read against the conventional 8–12 mEq/L reference range regardless of the patient's albumin level.
Derivation from albumin's charge
The 2.5 factor is not arbitrary — it falls out of albumin's physical chemistry. Albumin has a net negative charge of approximately −0.25 mEq per gram per deciliter at pH 7.4 (the role of albumin as the dominant buffer and unmeasured anion is covered in detail elsewhere on this site). At a normal albumin of 4.0 g/dL, that works out to about 4 × 0.25 = 1.0 mEq/L of charge — but the relevant contribution to the gap is larger because albumin also shifts the apparent reference range through the unmeasured anion pool. Empirically, clinical studies by Figge and colleagues established that each 1 g/dL change in albumin shifts the gap by roughly 2.5 mEq/L, and that empirical coefficient has become the standard in textbooks and at the bedside. The 4.0 g/dL anchor is the conventional midpoint of the adult reference range for albumin.
Why the factor is 2.5
Put plainly: for every 1 g/dL the albumin drops below 4.0, add 2.5 mEq/L to the observed gap. For every 1 g/dL it rises above 4.0, subtract 2.5 mEq/L. The arithmetic is symmetric because the equation is linear in albumin. A patient with albumin of 2.0 g/dL (a 2 g/dL deficit) gets 2 × 2.5 = 5 mEq/L added back. A patient with albumin of 5.0 g/dL (a 1 g/dL excess) gets 1 × 2.5 = 2.5 mEq/L subtracted. This symmetry is what makes the Figge equation robust across the full clinical range of albumin values, from severe hypoalbuminemia in cirrhosis or nephrotic syndrome to hemoconcentration in dehydration.
Worked examples
Hypoalbuminemia masking HAGMA (albumin 2.0 g/dL). Consider a cirrhotic ICU patient with an observed anion gap of 12 mEq/L — sitting at the top of the conventional 8–12 range and reading as normal. Serum albumin is 2.0 g/dL. Apply the Figge equation: corrected AG = 12 + 2.5 × (4.0 − 2.0) = 12 + 2.5 × 2.0 = 12 + 5.0 = 17 mEq/L. The corrected gap is clearly elevated, unmasking a high-anion-gap metabolic acidosis that the raw value understated by 5 mEq/L. Without the correction, the workup for lactate, ketones, or toxins would have been delayed by a false-normal result. Try this and other values in the corrected anion gap calculator.
High albumin narrowing the gap (albumin 5.0 g/dL). Now take a dehydrated patient with an observed gap of 15 mEq/L — mildly elevated by the conventional range. Serum albumin is 5.0 g/dL (1 g/dL above normal). Apply the equation: corrected AG = 15 + 2.5 × (4.0 − 5.0) = 15 + 2.5 × (−1.0) = 15 − 2.5 = 12.5 mEq/L. After correction the gap sits right at the upper bound of normal — the apparent mild elevation was partly an artifact of the high albumin concentrating the unmeasured anion pool. This is the reverse scenario: the correction prevents over-calling a gap that dehydration has inflated.
The correction matters most when albumin is below 3.0 g/dL, because the hidden gap can exceed 2–3 mEq/L — enough to flip a "normal" reading into a clearly elevated one. In ICU, cirrhosis, nephrotic syndrome, and severe malnutrition, the corrected gap should be the default reading, not the optional one.
The Figge equation and the 2.5 mEq/L per g/dL coefficient are reviewed against StatPearls, LITFL, and the original Figge et al. work on albumin's contribution to the anion gap. Always pair the corrected value with the clinical context and the reference interval supplied by the testing laboratory.