Anion gap

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A normal gap is 12 +/- 4 (i.e. 8 to 16). The anion gap is the difference between measured cations and measured anions in serum. This difference does not reflect a true disparity between positive and negative charges, given that serum actually is electrically neutral when all serum cations and anions are measured. Rather, the anion gap is a measurement artifact resulting from the fact that only certain cations and anions are routinely measured. Anion gap metabolic acidosis is secondary to the addition of endogenous or exogenous acid

  • AG = Na - (Cl + HCO3)
  • Normal Anion Gap = 12+/-4 (8-16)
  • 12-20 mEq/L when including K+

Cutoffs for "normal" Anion Gap are laboratory and equipment specific. Newer technology and equipment have been shown to measure "low" AG in otherwise normal, healthy people.[1][2]

Elevated Anion Gap

Differential Diagnosis




Low Anion Gap

Differential Diagnosis

  • AG falls by 2.5 meq/L for every 1 g/dL reduction in albumin concentration

See Also

External Links


  1. Jurado RL, del Rio C, Nassar G, Navarette J, Pimentel JL Jr. "Low anion gap." South Med J. 1998;91(7):624
  2. Winter SD, Pearson JR, Gabow PA, Schultz AL, Lepoff RB. "The fall of the serum anion gap." Arch Intern Med. 1990;150(2):311