Anion Gap

The serum anion gap is the difference between the concentrations of the major cation, sodium and the major anions(Cl + HCO₃). An electrolyte abnormality is often the first laboratory sign of an acid-base disorder. The anion gap measurement is likely to be most useful in emergency department in cases with ketoacidosis, lactic acidosis or renal failure.

Interpretation:

• Normal Values:
  5-14 mmol/L (Only values < 2 and > 17 mmol/L are likely to be clinically significant).
  
  Acidosis can occur even when the anion gap is normal, as in the following cases:
  • gastrointestinal HCO₃ loss; diarrhoea, fistulas etc.
  • increased Chloride; uterosigmoidostomy etc.
  • renal tubular acidosis
  • drugs, eg acetozolamide
  • administration of ammonium chloride
• Increased anion gap could indicate:
  • renal failure:
    • increase in phosphate, sulphate
    • diabetic ketoacidosis,starvation ketoacidosis
    • lactic acidosis
  • poisonings:
    • methanol - formic acid metabolite
    • ethylene glycol - oxalic/glycolic acids
    • aspirin - salicylic acid
  
  In practice, an increased anion gap is usually due to an accumulation of unmeasured anions.
• Decreased anion gap could indicate:
  • hypoalbuminemia.
  • marked increase in the cations.

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