How does albumin affect the anion gap?

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 rest (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/L, 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.

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.