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.
|Pathophysiology of acid-base disorders |
|Respiratory acidosis||Respiratory alkalosis||Metabolic acidosis||Metabolic alkalosis|
|pH*|| || || || |
|PCO2|| || || || |
|HCO3-|| || || || |
|Mechanism|| || || |
|Common causes|| || || |
- pH scale
- Henderson-Hasselbalch equation allows for calculation of HCO3- from pH and pCO2: pH = 6.1 + log ([HCO3-] / 0.03 x pCO2)
Assessment of acid-base status 
Start with anand then proceed in the following order:
- Evaluate blood pH:
Evaluate pCO2 (partial pressure of carbon dioxide in blood, reference range: 33–45 mm Hg) to determine whether the primary is respiratory or metabolic:
- pH and pCO2 change in the opposite direction: respiratory disorder
- pCO2 and pH change in the same direction: metabolic disorder
- Suspect a mixed acid-base disorder if:
- Evaluate HCO3- (reference range: 22–28 mEq/L):
- Evaluate pO2:
Compensation (acid-base) 
- Definition: physiological changes that occur in acid-base disorders in an attempt to maintain normal body pH
- Compensatory changes
|Compensation mechanisms in acid-base disorders|
|Primary disorder||Compensatory process||Expected compensation*|
|Respiratory acidosis||Acute compensation|| |
|Respiratory alkalosis||Acute compensation|| |
|*If the expected compensation does not occur, a secondary acid-base disturbance will be present in addition to the primary disorder.|
An anion gap represents the difference between the concentration of unmeasured anions and the concentration of unmeasured cations.
Anion gap = [Unmeasured anions] - [Unmeasured cations]
- [Unmeasured anions] = [Total anions] - [Routinely measured anions]
- [Unmeasured cations] = [Total cations] - [Routinely measured cations]
- Since maintenance of electrical neutrality requires that the total concentration of cations approximate that of anions, the anion gap formula can also be expressed as: anion gap = [Routinely measured cations] - [Routinely measured anions]
- If potassium concentration is normal, anion gap ≈ [Na+] - ([Cl-] + [HCO3-]) (reference range: 6–12 mmol/L)
- If potassium levels are also taken into consideration: anion gap = ([Na+] + [K+]) - ([Cl-] + [HCO3-]) (reference range: 10–16 mmol/L)
- An increase of unmeasured anions can increase the anion gap (see “” below).
Normal anion gap metabolic acidosis (also known as hyperchloremic acidosis)
- Primary loss of HCO3- compensated with ↑ Cl- → normal anion gap
- Further evaluation: urine anion gap = [urine Na+] + [urine K+] - [urine Cl-]
High anion gap metabolic acidosis
- Increased concentration of organic acids such as lactate, ketoacids (beta-hydroxybutyrate, acetoacetate), oxalic acid, formic acid, or glycolic acid; → no compensatory increase of Cl- → ↑ anion gap
- Further evaluation: delta ratio = (anion gap - 12) / (24 - [HCO3-]) 
- Delta ratio < 1 : A hyperchloremic or normal anion gap metabolic acidosis is present in addition to a high anion gap metabolic acidosis.
- Delta ratio 1–2 : Only a high anion gap metabolic acidosis is present.
- Delta ratio > 2 : A chronic respiratory acidosis or a metabolic alkalosis is present in addition to a high anion gap metabolic acidosis.
Abnormal anion gap without metabolic acidosis 
- Hypoalbuminemia → ↓ [Unmeasured anions] → ↓ anion gap
- Paraproteinemia (e.g., multiple myeloma), severe hypercalcemia, severe hypermagnesemia, and/or lithium intoxication → ↑ [Unmeasured cations] → ↓ anion gap
- Severe hyperphosphatemia → ↑ [Unmeasured anions] → ↑ anion gap 
- Severe hypocalcemia and/or hypomagnesemia → ↓ [Unmeasured cations] → ↑ anion gap
Causes of high anion gap acidosis (MUDPILES): Methanol intoxication, Uremia, Diabetic ketoacidosis, Paraldehyde, Isoniazid or Iron overdose, Inborn error of metabolism, Lactic acidosis, Ethylene glycol intoxication, Salicylate intoxication
Causes of normal anion gap acidosis (FUSEDCARS): Fistula (biliary, pancreatic), Ureterogastric conduit, Saline administration, Endocrine (Addison disease, hyper-PTH), Diarrhea, Carbonic anhydrase inhibitor, Ammonium chloride, Renal tubular acidosis, Spironolactone
Treatment of acid-base disorders should always address the underlying cause. Some steps in urgent management are listed below. 
- Respiratory acidosis: Treat underlying cause; (see “Treatment” in “COPD”, “Opioid intoxication”, and “Benzodiazepine overdose” articles).
- Respiratory alkalosis: Treat underlying cause.
- Acute severe metabolic acidosis: intravenous sodium bicarbonate 
- Chronic metabolic acidosis: oral sodium bicarbonate along with treatment of the underlying cause (e.g., diarrhea, renal tubular acidosis)
- Electrolyte disturbances: correction (e.g., see “Disorders of potassium balance”)
- See “Diabetic ketoacidosis” and “Salicylate toxicity”.
- Metabolic alkalosis