Lab
Anion Gap
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Anion Gap
, Corrected Anion Gap
See Also
Arterial Blood Gas
Arterial Blood Gas Interpretation
Acid-Base Homeostasis
Metabolic Acidosis with High Anion Gap
Definitions
Anion Gap
Difference between calculated serum anions and cations
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
Calculation
Anion Gap
AG =
Serum Sodium
-
Serum Chloride
- Serum Bicarbonate
AG = uAnions - uCations
Where uAnions = Unmeasured anions (e.g. Albumin, sulfate, phosphate,
Lactic Acid
,
Ketone
s)
Where uCations = Unmeasured cations (e.g.
Magnesium
, gamma globulins)
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
Where AlbAct is the measured, actual albumin
Interpretation
Normal Anion Gap: 12 +/- 2 meq/L
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
Causes
High Anion Gap
See
Metabolic Acidosis with High Anion Gap
(without increased
Serum Chloride
)
Severe alkalemia (albumin become negatively charged)
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
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