Anion Gap Acidosis & Delta-Delta Gap

1) Interpretation of Anion Gap Acidosis:
Remember that the expected AG is 2.5 x albumin  (so usually about 10 is normal).
Ddx: MUDPILES = Methanol, Uremia, DKA (but other ketoacidoses can do the same - e.g. alcoholic and starvation ketoacidosis), Paraldehyde/Propylene Glycol, Isoniazid, Lactic Acidosis, Ethylene Glycol, and Salicylates.  Most common cause in the hospital is lactic acidosis. 

2) Calculation of delta-delta gap: Easiest way to remember is to compare the delta AG (Calculated AG - Expected AG) to the delta HCO3 (24 - measured HCO3).  If the HCO3 has decreased more than expected from the anion gap, then there is a concurrent non-anion gap acidosis that is contributing to the lower bicarb.  If the HCO3 has decreased less than expected, then there is a concurrent metabolic alkalosis raising the bicarb.  In our patient, the delta delta was (30 - expected AG of 10) / (24 - HCO3 of 21) = 20/3 = 6.7.  For a ratio > 2, this indicates a concurrent metabolic alkalosis, likely due to vomiting and/or contraction alkalosis.

(Chanu Rhee MD, 6/7/10)