Anion Gap - Interpretation and Causes

Interpretation and Causes

Anion gap can be classified as either high, normal or, in rare cases, low. Laboratory errors need to be ruled out whenever anion gap calculations lead to results that do not fit the clinical picture. Methods used to determine the concentrations of some of the ions used to calculate the anion gap may be susceptible to very specific errors. For example, if the blood sample is not processed immediately after it is collected, continued leukocyte cellular metabolism may result in an increase in the HCO₃− concentration, and result in a corresponding mild reduction in the anion gap. In many situations, alterations in renal function (even if mild, e.g., as that caused by dehydration in a patient with diarrhea) may modify the anion gap that may be expected to arise in a particular pathological condition.

A high anion gap indicates that there is loss of HCO₃− without a concurrent increase in Cl−. Electroneutrality is maintained by the elevated levels of anions like lactate, beta-hydroxybutyrate and acetoacetate, PO₄−, and SO₄−. These anions are not part of the anion-gap calculation and therefore a high anion gap results. Thus, the presence of a high anion gap should result in a search for conditions that lead to an excess of these substances.