II. Definitions

  1. Anion Gap
    1. Difference between calculated serum anions and cations

III. Physiology

  1. Anion Gap is maintained by near balance of key cations (sNa+) and key anions (sCl-, sHCO3-)
  2. In Non-Anion Gap Metabolic Acidosis, only measured cations and anions are affected
    1. In Diarrhea, bicarbonate is lost and compensated by chloride increase
  3. In Anion Gap Metabolic Acidosis, unmeasured anions are increased
    1. Increased Lactic Acid or Ketoacids, for example, result in a significant Anion Gap

IV. Calculation: Anion Gap

  1. AG = Serum Sodium - Serum Chloride - Serum Bicarbonate
  2. AG = uAnions - uCations
    1. Where uAnions = Unmeasured anions (e.g. Albumin, sulfate, phosphate, Lactic Acid, Ketones)
    2. Where uCations = Unmeasured cations (e.g. Magnesium, gamma globulins)

V. Calculation: Corrected Anion Gap (severe hypoalbuminemia)

  1. Corrected Anion Gap indicated when Serum Albumin is very low
  2. AGcorr = AGact + 2.5 * (AlbNl - AlbAct)
    1. Where AGcorr is Anion Gap corrected
    2. Where AGact is the measured, actual Anion Gap
    3. Where AlbNl is the normal albumin
    4. Where AlbAct is the measured, actual albumin

VI. Interpretation

  1. Normal Anion Gap: 12 +/- 2 meq/L

VII. Causes: Low Anion Gap

  1. Paraproteinemia (Multiple Myeloma)
  2. Spurious Hyperchloremia (Bromide toxicity)
  3. Hyponatremia
  4. Hypermagnesemia
  5. Hypoalbuminemia
    1. See Corrected Anion Gap above
    2. Anion Gap decreases 2.5 meq per 1 g/dl Albumin drop

VIII. Causes: High Anion Gap

  1. See Metabolic Acidosis with High Anion Gap (without increased Serum Chloride)
  2. Severe alkalemia (albumin become negatively charged)

IX. References

  1. Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114
  2. Bakerman (1984) ABCs of Lab Data, ILD, Greenville, NC
  3. Ghosh (2000) Fed Pract p. 23-33

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Related Studies

Ontology: Anion Gap (C0003074)

Concepts Clinical Attribute (T201)
MSH D000136
SnomedCT 102637000
LNC LP30809-5, MTHU015314
English Anion Gaps, Gap, Anion, Gaps, Anion, Anion gap, function, anion gap, AG - Anion gap, Anion gap, Anion gap, function (observable entity), Anion gap (function), Anion Gap
Portuguese Gap Aniônico
Spanish Anión Gap, anión gap, anion gap, anión restante, brecha aniónica (entidad observable), brecha aniónica (función), brecha aniónica, hiato aniónico, intervalo aniónico
French Trou anionique
German Anionen-Lücke
Italian Gap anionico
Czech AG, anion gap, aniontová mezera
Norwegian Anion gap
Dutch Anion gap