Lab

Anion Gap

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Anion Gap, Corrected Anion Gap

  • Definitions
  1. Anion Gap
    1. Difference between calculated serum anions and cations
  • 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
  • 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)
  • 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
  • Interpretation
  1. Normal Anion Gap: 12 +/- 2 meq/L
  • 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
  • Causes
  • High Anion Gap
  1. See Metabolic Acidosis with High Anion Gap (without increased Serum Chloride)
  2. Severe alkalemia (albumin become negatively charged)
  • 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