Contributor: Dylan Luyten, MD Educational Pearls Acidosis can be caused by a respiratory, metabolic, or mixed source A respiratory acidosis will have a low serum pH and elevated bicarbonate if it’s chronic, but most importantly end tidal CO2 or arterial CO2 will be high.   With a metabolic acidosis we expect to see a low serum pH and low serum bicarbonate. Once it’s determined a patient is in a metabolic acidosis, we need to determine if it’s an elevated anion gap or normal anion gap acidosis. Anion gap is calculated by subtracting the serum anions from the cations, or serum sodium minus the serum chloride and CO2 (Na-(Cl+CO2)). A normal gap is 6-10, gaps over 10 have an elevated anion gap acidosis. The causes of an elevated anion gap acidosis can be remembered with MUDPILES mnemonic (Methanol, Uremia (renal failure), Diabetes/DKA, Paraldehyde, Isoniazid, Lactate, Ethanol/Ethylene glycol, salicylates/aspirin). So when you suspect an elevated gap acidosis you need to evaluate for all causes, so you need to get an venous or arterial blood gas along with lactate, salicylate, and Tylenol levels. References Vanmassenhove J. Lameire N. Approach to the patient presenting with metabolic acidosis. Acta Clin Belg. 2019 Feb;74(1):21-27. doi: 10.1080/17843286.2018.1547245. Epub 2018 Nov 24. Burger MK, Schaller DJ. Physiology, Acidosis, Metabolic. [Updated 2019 Jun 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482146/   Summarized by Jackson Roos, MS3 | Edited by Erik Verzemnieks, MD