Normal values (reference ranges):
- 16 +- 4 mEq/L (if potassium is used in the calculation)
- 12 +- 4 mEq/L (if potassium is not used in the calculation)
The anion gap (AG) is the difference between the cations and the anions in the extracellular space that are routinely calculated in the laboratory.
Anion Gap = [Sodium + Potassium] – [Chloride + Bicarbonate]
In some labs, the potassium is not measured because the level of potassium in acid-base abnormalities varies. The normal value of the anion gap is adjusted downward if potassium is removed from the equation. The anion gap, although not really physiologically, is created by the small amount of anions in the blood:
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- lactate
- phosphates
- sulfates
- organic anions
- proteins
The calculation is most often helpful in identifying the cause of metabolic acidosis. As acids such as lactic acid or ketoacids accumulate in the bloodstream, bicarbonate neutralizes them to maintain a normal pH within the blood.
Mathematically, when bicarbonate decreases, the AG increases. In general, most metabolic acidotic states are associated with an increased anion gap. The higher the gap above normal, the more likely this will be the case. Proteins can have a significant effect on AG. As albumin (usually negatively charged) increases, AG will increase.
A decreased AG is very rare but can occur when there is an increase in unmeasured cations (calcium or magnesium). A reduction in anionic proteins (nephrotic syndrome) will also decrease AG.
To find out more about Anion Gap and what it means if your result values are too high or too low, please visit this page: Anion Gap
