Anion Gap
The anion gap (AG) is calculated from measured results by the following formula:
AG = (Na++K+)-(Cl-+
HCO3-)
Na+ and K+ account for about 95% of total serum cations while Cl- and HCO3 account for about 85% of the total serum anions in a healthy individual. The
"normal" anion gap is due to the presence of various unmeasured anions, e.g. plasma proteins,
phosphate, and sulfate. An increased anion gap indicates the presence of excessive
amounts of one or more of these anions or of some other anion.
The formula shown above is derived from the following equations:
- To maintain electrical neutrality, anions must equal cations. This can be expressed
as
Na+ +K+ +UC = Cl- +HCO3- +UA
Unmeasured cations (UC): Calcium, magnesium, and gamma globulins. These are rarely ever increased enough to decrease the anion gap.
Unmeasured
anions (UA) : Lactate, phosphate, sulfate, ketones, metabolites of some poisons, such as ethylene glycol, plasma proteins.
- Rearrangement of the equation above gives -
UA-UC = (Na++K+)-(Cl-+HCO3-)
Anion gap = (UA-UC)
- Since UC cannot change enough to affect the anion gap and still be compatible with
life, UC can be dropped from the equation. Thus,
Anion gap = UA = (Na++K+)-(Cl-+HCO3-)
As can be seen from these equations, the AG is a figure that indicates the amount of
unmeasured anions in serum expressed in mEq/L.
Increased anion gap
- Titration acidosis: Most common cause of an increased anion gap.
- Alkalemia: loss of protons from plasma proteins (unmeasured anions) increases their negative charge. Alkalemia also stimulates lactic acid production. The increase in AG is very mild.
- Dehydration: Will increase plasma protein concentration (especially albumin).
- Sodium containing drugs: Sodium salts, penicillin.
- Age: Anion gap is higher in foals than adult horses.
- Decreased cations: magnesium and calcium. Have minimal affect on the anion gap because of their low concentrations.
Decreased anion gap
- Acidemia: Causes dissociation of protons from plasma proteins, decreasing their negative charge.
- Decreased albumin: A very common cause of a lower than expected or decreased anion gap.
- Assay artefacts: Artefactually elevated chloride, e.g. bromide therapy.
- Dilution: Dilutes plasma proteins.
- Increased unmeasured cations: Calcium, magnesium, gamma globulins, lithium. (These rarely cause an increased anion gap as most increases are incompatible with life. It is unusual to see a low anion gap in multiple myeloma.)
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