• Clinical science

Acid-base disorders


Acid-base disorders are a group of conditions characterized by changes in the concentration of hydrogen ions (H+) or bicarbonate (HCO3-), which lead to changes in the arterial blood pH. These conditions can be categorized as acidoses or alkaloses and have a respiratory or metabolic origin, depending on the cause of the imbalance. Diagnosis is made by arterial blood gas (ABG) interpretation. In the setting of metabolic acidosis, calculation of the anion gap is an important resource to narrow down the possible causes and reach a precise diagnosis. Treatment is based on identifying the underlying cause.


Respiratory acidosis Respiratory alkalosis Metabolic acidosis Metabolic alkalosis
PCO2 ↓ (compensation) ↑ (compensation)
HCO3- ↑ (compensation) ↓ (compensation)
  • ↑ Production/ingestion of H+ or loss of HCO3-
  • Loss of H+ or ↑ production/ingestion of HCO3-
Common causes

*pH values may be within the reference range in the case of competely compensated alkalosis or acidosis.



SMORE: change of pCO2 in the Same direction as pH Metabolic disorder; change of pCO2 in the Opposite direction of pH REspiratory disorder


  • Definition: physiological changes that occur in acid-base disorders in an attempt to maintain normal body pH
  • Compensatory changes
Primary disorder Compensatory process Expected compensation*
Metabolic acidosis
Metabolic alkalosis
  • Expected pCO2 = (0.7 x HCO3-) + 20 (+/- 5)
Respiratory acidosis Acute compensation
  • Buffers in blood
  • Expected HCO3- = 24 + [0.1 x (pCO2 - 40)](+/- 3)
Chronic compensation
  • Expected HCO3- = 24 + [0.4 x (pCO2 - 40)] (+/- 3)
Respiratory alkalosis Acute compensation
  • Buffers in blood
  • Expected HCO3- = 24 - [0.2 x (40 - pCO2)] (+/- 3)
Chronic compensation
  • Expected HCO3- = 24 - [0.5 x (40 - pCO2)] (+/- 3)
*If the expected compensation does not occur, a secondary acid-base disturbance will be present in addition to the primary disorder


Anion gap

Causes of high anion gap acidosis (MUDPILES): Mmethanol intoxication, Uuremia, D – diabetic ketoacidosis, P paraldehyde, I isoniazid or iron overdose, inborn error of metabolism, L – lactic acidosis, E – ethylene glycol intoxication, Ssalicylate intoxication

Causes of normal anion gap acidosis (FUSEDCARS): F Fistula (biliary, pancreatic), U ureterogastric conduit, S saline administration, E endocrine (Addison's disease, hyper-PTH), D diarrhea, C carbonic anhydrase inhibitor, A ammonium chloride, R renal tubular acidosis, S spironolactone

A neGUTive urine anion gap implies GI loss of bicarbonate.



Treatment of acid-base disorders should always address the underlying cause. Some steps in urgent management are listed below.