The anion gap is a calculated value derived from the serum electrolytes in the following manner:
The normal value ranges from 12-15 mEq/L. Extremely low values are seen in multiple myeloma but are not diagnostic. The anion gap is used primarily in determining the cause of a metabolic acidosis as the differential diagnosis for a normal anion gap versus a high anion gap acidosis is markedly different. The anion gap is low in multiple myeloma, hypoalbuminemia, hypermagnesemia, hyperkalemia, hypercalcemia, and hyponatremia. Hypoalbuminemia is a frequent lab abnormality among hospitalized patients, and the anion gap must be corrected in its presence. When the serum albumin is decreased by one-half, the anion gap will be falsely decreased by approximately 6 mEq/L. Therefore, the calculated anion gap in a patient with significant hypoalbuminemia should be corrected by adding 6 mEq/L to the calculated value.
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