The normal anion gap is 8–12 mEq/L (with albumin at 4 g/dL). For every 1 g/dL fall in albumin below 4 g/dL, the expected AG falls by approximately 2.5 mEq/L — making hypoalbuminaemia the commonest cause of a spuriously normal AG.
| Delta Ratio | Interpretation |
|---|---|
| <0.4 | Normal AG acidosis / hyperchloraemic metabolic acidosis |
| 0.4–0.9 | Mixed high-AG + normal-AG metabolic acidosis |
| 1.0–2.0 | Pure high-AG metabolic acidosis (expected) |
| >2.0 | High-AG acidosis + concurrent metabolic alkalosis |
The delta ratio is only meaningful when the corrected AG is elevated (>12 mEq/L). It compares the rise in AG above normal against the fall in bicarbonate below normal to identify co-existing disorders.
When to correct for albumin: Always use the Figge correction in critically ill patients, those with liver disease, nephrotic syndrome, or malnutrition — all conditions where hypoalbuminaemia is prevalent. A "normal" raw AG in a hypoalbuminaemic patient may conceal a significant unmeasured anion load.
Approach to mixed acid-base disorders: Once an elevated corrected AG is confirmed, apply the delta ratio. A ratio <1 suggests a concomitant normal-AG acidosis (e.g., diarrhoea + DKA). A ratio >2 suggests a concurrent metabolic alkalosis (e.g., vomiting + DKA, or diuretic use).
Limitations: The delta ratio assumes a baseline AG of 12 and baseline HCO₃ of 24. Chronic respiratory compensation will alter HCO₃; adjust interpretation accordingly. The ratio is unreliable in pure lactic acidosis where lactate is rapidly cleared.