In evaluating metabolic acidosis, what does an elevated anion gap with a delta ratio greater than 2 indicate?

• A. A pure high anion gap metabolic acidosis with no other disturbances.
• B. Coexistence of a concurrent non-anion gap metabolic acidosis or other unmeasured anions; a mixed acid-base disorder is present.
• C. A measurement error in electrolyte testing.
• D. Primary respiratory alkalosis.

Answer: (B)

The concept being tested is how to interpret the delta ratio when an elevated anion gap metabolic acidosis is present. The delta ratio compares how much the anion gap has increased to how much the bicarbonate has decreased.

In a simple elevated anion gap acidosis, the increase in unmeasured anions (raising the gap) tracks with the drop in bicarbonate; the delta ratio is around 1. If the delta ratio is greater than about 2, the rise in the anion gap is disproportionately large compared with the drop in bicarbonate. That means there is another acid-base process happening besides the elevated anion gap acidosis—most commonly a concurrent non-anion gap metabolic acidosis (hyperchloremic acidosis) or other unmeasured anions contributing to the overall picture.

So, an elevated anion gap with a delta ratio > 2 points to a mixed acid-base disorder, specifically coexistence of a non-anion gap metabolic acidosis or additional unmeasured anions.