The Anion Gap
March 4th, 2023
Overview
The anion gap is used to further assess a patient with metabolic acidosis.
It is a calculation of the unmeasured anions and cations in the blood, based on anions and cations that we can measure (sodium, chloride and bicarbonate). This assumes that the body is electrochemically neutral, i.e.:
Anions = Cations
Or in other words:
Na⁺ + unmeasured cations = HCO₃⁻ + Cl⁻ + unmeasured anions
Therefore, the anion gap is the "gap" between measured cations and measured anions, suggesting the amount of unmeasured cations and anions present:
Anion Gap = Na⁺ - ( Cl⁻ + HCO₃⁻ )
If there is significant elevation of the anion gap then this suggests that there is an abundance of unmeasured ion present - usually an acid such as lactic acid or ketones.
Some calculations also take into account potassium, however this is not a major contributor and is more often than not left out.
Normal Range
- 8 - 16
Albumin, an anion, makes up ~10mmol/L of the anion gap. The above formula assumes that the albumin is normal, thus it is important to check that the patient is not hypoalbuminaemic when calculating the anion gap.
Normal Anion Gap Metabolic Acidosis
A normal anion gap of 8 - 16 suggests that there is loss of bicarbonate as the cause of the patient's acidosis, either renally or due to GI loss.
Causes of Normal Anion Gap Metabolic Acidosis
- GI loss - diarrhoea, fistula, stoma
- Isotonic saline infusion
- Renal tubular acidosis
- Acetazolamide
- Addison's disease
High Anion Gap Metabolic Acidosis
An elevated anion gap of >16 suggests that there is an excess of acid within the blood, causing an acidosis.
Causes of High Anion Gap Metabolic Acidosis
- Lactic acidosis
- Ketoacidosis - diabetic, alcoholic, starvation
- Uraemia (end-stage renal failure)
- Methanol / ethanol / ethylene glycol
- Salicylate toxicity
- Carbon monoxide toxicity
The Gap-Gap Ratio can be used to determine whether the high anion gap metabolic acidosis is the sole cause of the patient's metabolic acidosis, or if there is a coexistant normal anion gap metabolic acidosis.
Reduced Anion Gap
A low anion gap of <8 suggests that there is some unmeasured ion present in abundance.
Causes of Reduced Anion Gap
- Decreased unmeasured anions - albumin
- Increased unmeasured cations - calcium, magnesium, lithium
- Lab error
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