Back to Basics: Anion Gap Metabolic Acidosis

Anion Gap Metabolic Acidosis


Basic Calculation of the Anion Gap: Na - (Cl + HCO3)


Normal Value: 12 +/- 4


Causes of Elevation - due to unmeasured anions

Many causes, 2 mnemonics below account for the more common causes in clinical practice:


MUD PILES: the classic mnemonic



Uric acid

Diabetic ketoacidosis (also alcoholic or starvation ketoacidosis)


Paraldehyde, paracetamol, propylene glycol

Isoniazid, Iron


Ethylene glycol




A sample case:

A 55 year old male presents to the ED found to have acute renal failure. Lab results as follows:

Na 139, K 4.3, Cl 102, CO2 10, Cr 11.14, BUN 84, glucose 122, Lactate 1.5. 


Calculate the anion gap?

139-10-102 = 27


Why is his gap elevated?


Likely due to elevation of uric acid level in setting of acute renal failure. Or, could he have ingested something that caused both his renal failure and a elevated anion gap? You'll have to decide. Take a good history, and consider ordering a serum osm level and calculating an osmolar gap as well!


Don't like MUDPILES?

Here's another mnemonic option you may not have heard of



Glycols (ethylene or propylene glycol)

Oxoproline (from chronic acetaminophen/paracetamol ingestion)


D-lactate (we don't measure this much)




Renal Failure (uremia)


Ketoacidosis (diabetic, starvation, alcoholic)




Kelen, G, Nicolaou, D, Cline, D. “Acid-Base Disorders.” Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8eEds. Judith E. Tintinalli, et al. New York, NY: McGraw-Hill, 2016.


Mehta, Ankit N; Emmett, Joshua B; Emmett, Michael (2008). "GOLD MARK: An anion gap mnemonic for the 21st century". The Lancet372 (9642): 892​