• In metabolic acidosis, pH is low, plasma bicarbonate is low and the anion gap is normal or increased.
• The anion gap is the calculated difference between the measured cations (Na+ and K+) and the measured anions (HCO3- and Cl-).
• An increased anion gap metabolic acidosis, indicates the accumulation of unmeasured anions and, therefore, the accumulation of an acid species, such as ketoacids or lactic acid.
• As electroneutrality is always maintained, the level of cations (potassium and sodium ions) must be increased, resulting in a raised anion gap.
• A normal anion gap metabolic acidosis indicates that the primary effect of the acidosis has been a loss of bicarbonate.
• This is balanced by accumulation of chloride to maintain electroneutrality and as both bicarbonate and chloride are measured, the anion gap is not altered.

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Hyperkalemic hyperchloremic metabolic acidosis: Pathophysiologic insights
DuBose TD, 1997
Kidney International 51 591-602

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C. A. O'Callaghan 2004