How does albumin affect the anion gap?
Albumin is the dominant "unmeasured" anion sustaining the normal anion gap, so when albumin falls the anion gap falls with it — which is why hypoalbuminemia is the single most common reason a high anion gap metabolic acidosis is missed.
Albumin is ~75% of the normal anion gap
Although the anion gap formula Na⁺ − (Cl⁻ + HCO₃⁻) looks like it measures only three ions, the gap it produces is sustained almost entirely by the body's unmeasured anions — and albumin is by far the largest of these. Albumin is a negatively charged plasma protein, and at a normal serum concentration of ~4.0 g/dL it contributes roughly three-quarters of the entire normal anion gap; phosphate, sulfate, and trace organic anions make up the rest12 (Figge et al., Critical Care Medicine; Kraut & Madias, StatPearls 'Serum Anion Gap').
The clinical consequence follows directly: if albumin falls, the anion gap falls. Each 1 g/dL decrease in serum albumin lowers the expected anion gap by approximately 2.5 mEq/L1, so a patient with albumin of 2.0 g/dL has a "normal" baseline gap near 4 mEq/L rather than 12 — purely from protein loss. Because the missing albumin pulls the gap downward at the same time that accumulating acid anions (lactate, ketones) push it upward, the two effects cancel and a genuine high-anion-gap metabolic acidosis (HAGMA) can hide behind a deceptively "normal" number. This masked HAGMA is one of the classic missed diagnoses in critical care.
The fix is the albumin-corrected anion gap (the Figge equation). For the underlying biochemistry, see our page on the role of albumin; for the formula and its derivation, see the corrected anion gap formula page; and for the clinical when and why, see why albumin matters in interpretation.
In ICU, liver disease, nephrotic syndrome, severe malnutrition, or any patient with albumin < 3.0 g/dL, never interpret an uncorrected anion gap. Run the corrected anion gap calculator instead.
Key takeaways
- Albumin accounts for ~75% of the normal anion gap.
- Each 1 g/dL fall in albumin lowers the expected AG by ~2.5 mEq/L.
- Hypoalbuminemia masks HAGMA — the most common cause of a missed high anion gap metabolic acidosis.
- Always apply the Figge correction in critically ill, cirrhotic, nephrotic, or malnourished patients.
References
- Figge J, et al. "The role of serum proteins in acid-base equilibria." J Lab Clin Med. 1998;117(6):453-467. (Albumin is ~75% of the normal AG; correction 2.5 mEq/L per 1 g/dL; Figge equation.) See also StatPearls — Biochemistry, Anion Gap. Link ↗
- Kraut JA, Madias NE. "Serum Anion Gap: Its Uses and Limitations in Clinical Medicine." Clin J Am Soc Nephrol. 2007;2(1):162-174. Link ↗
- Life in the Fast Lane (LITFL). "Anion Gap." CCC Acid-Base. (Hypoalbuminemia masks HAGMA in ICU.) Link ↗
- MDCalc. "Anion Gap." (Albumin-corrected AG calculator; 2.5 factor.) Link ↗
Don't let low albumin hide the gap.
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