A simple high anion gap metabolic acidosis produces a Delta_Delta around 0.

A Delta_Delta significantly greater than 0 usually indicates:

• A mixed high anion gap metabolic acidosis plus a primary metaboloc alkalosis.
• A mixed high anion gap metabolic acidosis plus a chronic respiratory acidosis.
• A non-acidotic high anion gap state (such as with excess penicillin administration.)

A Delta_Delta significantly less than 0 may result from:

• A mixed high anion gap metabolic acidosis plus a normal anion gap metabolic acidosis.
• A mixture of high anion gap metabolic acidosis plus chronic respiratory alkalosis plus hyperchloremia acidosis.
• A high anion gap acidosis with a prexisting low anion gap state (such as hypoalbuminemia, para-proteinemia, etc ...)

References

Wrenn K, The delta (delta) gap: an approach to mixed acid-base disorders, Ann Emerg Med, 1990 Nov;19(11):1310-3, PubMed ID number 2240729

Breyer JA, Acid base course material, 6th National Critical Care Medicine Review Course