Low anion gap

South Med J. 1998 Jul;91(7):624-9. doi: 10.1097/00007611-199807000-00004.


Background: The purpose of this review is to provide a differential diagnosis for a low anion gap.

Methods: We describe the anatomy of the anion gap. Also, we follow-up with a review of the English-language literature describing the causes of a low anion gap. During the past 15 years, the introduction and widespread clinical use of ion-selective electrode methodology for measuring serum electrolyte values has caused a major fall in the normal range of the anion gap from 12 mEq/L +/- 4 mEq/L to 6 mEq/L +/- 3 mEq/L; therefore, a new definition for a low anion gap is in order.

Results: Based on current clinical data, an anion gap value of < 3 mEq/L should be considered low. A low anion gap is a useful diagnostic tool, but its clinical significance is often unrecognized. Also, it may be a handy clinical clue for the diagnosis of life-threatening intoxications or occult neoplasms, such as multiple myeloma. The baseline low anion gap may mask the identification of a high gap metabolic acidosis in certain patients.

Conclusions: Interpretation of a low anion gap can provide valuable clinical information.

Publication types

  • Review

MeSH terms

  • Acid-Base Equilibrium*
  • Acid-Base Imbalance / blood*
  • Acid-Base Imbalance / diagnosis*
  • Acid-Base Imbalance / etiology
  • Bias
  • Diagnosis, Differential
  • Electrolytes / blood
  • Humans
  • Ion-Selective Electrodes
  • Reference Values
  • Reproducibility of Results


  • Electrolytes