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Albumin-Corrected Anion Gap Calculator (Figge)
Albumin is the largest unmeasured anion in plasma, so a low albumin lowers the apparent anion gap and can hide a real high-anion-gap metabolic acidosis — which is why the albumin-corrected gap is the single most important refinement in the acid-base workup. This calculator applies the Figge correction directly: corrected AG = AG + 2.5 × (4.0 − albumin g/dL), on top of the base gap of Na − (Cl + HCO₃). Take a panel of sodium 140, chloride 104 and bicarbonate 24 with an albumin of 2.0 g/dL: the raw gap is 140 − (104 + 24) = 12 mmol/L, which looks reassuringly normal, but corrects to 12 + 2.5 × (4.0 − 2.0) = 17 mmol/L — a hidden high-anion-gap acidosis the uncorrected value completely misses. Enter the electrolytes and albumin to see both the raw and corrected gap side by side, with a lab-range flag. The normal range is lab-dependent (classically 8–12, ~3–11 on modern analyzers). This is an interpretation aid, not a diagnosis — verify clinically. Free, private, and computed entirely in your browser.
corrected AG = AG + 2.5 × (4.0 − albumin g/dL) — Figge
- Worked example: AG 12 at albumin 2.0 → corrected 17 mmol/L (hidden acidosis)
- Shown side by side with the raw gap and the lab-range flag
Frequently asked questions
How do you correct the anion gap for albumin?
Use the Figge correction: corrected AG = AG + 2.5 × (4.0 − albumin in g/dL). Each 1 g/dL that albumin falls below 4.0 hides roughly 2.5 mmol/L of anion gap. For an anion gap of 12 with an albumin of 2.0 g/dL, the correction gives 12 + 2.5 × 2.0 = 17 mmol/L — a raised gap the uncorrected value misses.
Why does low albumin lower the anion gap?
Albumin is the largest unmeasured anion in plasma, and the anion gap represents unmeasured anions. When albumin is low (hypoalbuminemia, common in critically ill and nephrotic patients), there are fewer unmeasured anions, so the apparent gap drops — which can mask a genuine high-anion-gap acidosis. Correcting for albumin restores the true gap. It is an interpretation aid, not a diagnosis.