Lab

Anion Gap

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Anion Gap, Corrected Anion Gap

  • See Also
  1. Acid-Base Homeostasis
  2. Metabolic Acidosis with high Anion Gap
  • Definition
  1. Difference between calculated serum anions and cations
  • 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, organic)
    2. Where uCations = Unmeasured cations (e.g. Potassium, calcium, 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. 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