Albumin-Corrected Anion Gap (Figge)
Albumin-Corrected Anion Gap (Figge) - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
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What this is
Apply the Figge correction to the anion gap to account for hypoalbuminemia. Albumin is the dominant unmeasured anion in serum; for every 1 g/dL the albumin sits below 4.0, the measured anion gap underestimates the real gap by about 2.5 mEq/L. Sophie Well computes the measured AG = Na - (Cl + HCO3), then returns the albumin-corrected AG so that a high-anion-gap metabolic acidosis (HAGMA) that has been masked by a low albumin becomes visible.
When to use it
Use this whenever the measured anion gap is normal or borderline in a patient whose albumin is low (cirrhosis, nephrotic syndrome, malnutrition, critical illness, sepsis) - a population that is over-represented in the ICU and on the medicine service, and in whom an unrecognized HAGMA changes the differential and the resuscitation plan. The correction is not magic: it only adjusts for albumin, so other unmeasured cations (lithium, hypermagnesemia, hypercalcemia) and unmeasured anions (paraproteins) can still mislead. If the corrected gap is elevated, run the standard HAGMA workup (lactate, ketones, renal panel, osmolal gap, toxic ingestion screen).
References
Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26(11):1807-1810. AG correction = +2.5 mEq/L per 1 g/dL drop in albumin from 4.0.
Worked example: Measured AG 10 (looks normal); corrected AG 15 (elevated -- consider HAGMA workup).
Sophie Well is a reference and educational tool. Not medical, legal, or financial advice. Does not replace clinician judgment, professional billing review, or legal counsel.