interpret an ABG

Interpreting an ABG is something you’ll have to do on a daily basis in the MICU. The most important thing is to have a method that you like and can remember, and to use it every time you review an ABG. The handout on this page explains an easy-to-use method, but it’s not the only way and certainly doesn’t claim to be the best.


There are three basic steps to ABG interpretation:

  1. Determine the primary disorder (respiratory or metabolic, alkalosis or acidosis).

  2. See if the compensation is adequate or inadequate; if it’s not adequate, then there is another disorder present.

  3. Use the Anion Gap to see if an organic acidosis is present. If the anion gap is higher than normal, then a metabolic organic acidosis must be happening.


Common Causes of Respiratory Acidosis

Drug overdose

COPD exacerbation


Septic shock

Neuromuscular disease


Common Causes of Respiratory Alkalosis

Pain and anxiety



Pulmonary embolism


Common Causes of Metabolic Alkalosis

Intravascular volume depletion

Vomiting or nasogastric suctioning



Common Causes of Metabolic Acidosis

(with an elevated anion gap)




Diabetic ketoacidosis


Iron, Isoniazid

Lactic acidosis

Ethylene glycol, Ethanol-related ketoacidosis

Salicylates, Starvation ketoacidosis, Sepsis


Common Causes of Metabolic Acidosis

(without an elevated anion gap)


Hyperchloremia, Hyperalimentation

Acetazolamide, Addison’s disease

Renal tubular acidosis



Ureteral diversion (ureterosigmoidostomy, ureterostomy)

Pancreatic problems (pancreaticoduodenal fistula, pancreatic pseudocyst)