Teaching Instructions: You may replicate this on a white board by drawing in the diagram as you walk through it.
After identifying a metabolic acidosis, calculate whether there is an ↑ anion gap (AG) (Step 3). Normal AG is Na – (HCO3 + Cl) and is ~ 8-12, however, is dependent on albumin. Because the uncalculated anions are predominantly albumin, patients with hypoalbuminemia have lower expected AGs. Thus, an AG of 12 may be elevated in a patient with low albumin.

The differential of AG metabolic acidosis (AGMA) is captured in the mneumonic MUDPILES. Other causes include cyanide or CO poisoning, meds (iron, zidovudine), rhabdomyolysis. Of note, toxic alcohols – methanol and ethylene glycol – will cause both an osmolar gap and elevated AG.
Patients with an AGMA should be evaluated for anothe concurrent metabolic process by calculating the delta-delta (ΔΔ) or our preferred method, the “residual HCO3“. Calculate the ΔAG and add this back to the measured HCO3, essentially accounting for the AGMA. If this residual HCO3 is normal (22-26), there is a pure AG because accounting for the AG fully corrected the metabolic process. If the residual HCO3 is low (>22), this indicates another metabolic acidosis is present even after accounting for the AGMA. Thus, there is a concurrent AGMA and NAGMA.