II. Definitions
- Anion Gap
- Difference between calculated serum anions and cations
III. Physiology
- Anion Gap is maintained by near balance of key cations (sNa+) and key anions (sCl-, sHCO3-)
- In Non-Anion Gap Metabolic Acidosis, only measured cations and anions are affected
- In Diarrhea, bicarbonate is lost and compensated by chloride increase
- In Anion Gap Metabolic Acidosis, unmeasured anions are increased
- Increased Lactic Acid or Ketoacids, for example, result in a significant Anion Gap
IV. Calculation: Anion Gap
- AG = Serum Sodium - Serum Chloride - Serum Bicarbonate
- AG = uAnions - uCations
- Where uAnions = Unmeasured anions (e.g. Albumin, sulfate, phosphate, Lactic Acid, Ketones)
- Where uCations = Unmeasured cations (e.g. Magnesium, gamma globulins)
V. Calculation: Corrected Anion Gap (severe hypoalbuminemia)
- Corrected Anion Gap indicated when Serum Albumin is very low
- AGcorr = AGact + 2.5 * (AlbNl - AlbAct)
- Where AGcorr is Anion Gap corrected
- Where AGact is the measured, actual Anion Gap
- Where AlbNl is the normal albumin (4 g/dl)
- Where AlbAct is the measured, actual albumin
VI. Interpretation
- Normal Anion Gap: 12 +/- 2 meq/L
VII. Causes: Low Anion Gap
- Paraproteinemia (Multiple Myeloma)
- Spurious Hyperchloremia (Bromide toxicity)
- Hyponatremia
- Hypermagnesemia
- Hypoalbuminemia
- See Corrected Anion Gap above
- Anion Gap decreases 2.5 meq per 1 g/dl Albumin drop
VIII. Causes: High Anion Gap
- See Metabolic Acidosis with High Anion Gap (without increased Serum Chloride)
- Severe alkalemia (albumin become negatively charged)
IX. References
- Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114
- Bakerman (1984) ABCs of Lab Data, ILD, Greenville, NC
- Ghosh (2000) Fed Pract p. 23-33