Total concentrations of cations and anions are normally equal, making serum electrically neutral. Measuring the gap between measured cation and anion levels provides information about the level of anions (including sulfate, phosphate, organic acids such as ketone bodies and lactic acid, and proteins) that are not routinely measured in laboratory tests. In metabolic acidosis, measuring the anion gap helps to identity the type of acidosis and possible causes. Further tests are usually needed to determine the specific cause of metabolic acidosis.
Procedure and posttest care
A normal anion gap does not rule out metabolic acidosis. It may occur in hyperchloremic acidoses, renal tubular acidosis, and severe bicarbonate-wasting conditions, such as biliary or pancreatic fistulas and poorly functioning ileal loops.
When acidosis results from loss of bicarbonate in the urine or other body fluids, the anion gap remains unchanged. This is known as normal anion gap acidosis.
An increased anion gap indicates an increase in one or more of the unmeasured anions (sulfate, phosphates, organic acids such as ketone bodies and lactic acid, and proteins). This may occur with acidoses that are characterized by excessive organic or inorganic acids, such as lactic acidosis or ketoacidosis.
When acidosis results from an accumulation of metabolic acids - as occurs in lactic acidosis, for example the anion gap increases (above 12 mEq/L) with the increase in unmeasured anions. Metabolic acidosis caused by such an accumulation is known as high anion gap acidosis.
A decreased anion gap is rare but may occur with hypermagnesemia and paraproteinemic states, such as multiple myeloma and Waldenstrom's macroglobulinemia.
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