Anion gap

Difference between certain measured cations and the measured anions in serum, plasma, or urine

The anion gap (AG or AGAP) is a value calculated from the results of multiple individual medical lab tests. It may be reported with the results of an electrolyte panel, which is often performed as part of a comprehensive metabolic panel. The anion gap is the quantity difference between cations (positively charged ions) and anions (negatively charged ions) in serum, plasma, or urine. The magnitude of this difference (i. e., "gap") in the serum is calculated to identify metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed. The term "anion gap" usually implies "serum anion gap", but the urine anion gap is also a clinically useful measure.

The anion gap is a calculated measure. It is computed with a formula that uses the results of several individual lab tests, each of which measures the concentration of a specific anion or cation. The concentrations are expressed in units of milliequivalents/liter (mEq/L) or in millimoles/litre (mmol/L).

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 cellular metabolism by leukocytes (also known as white blood cells) may result in an increase in the HCO 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 increased concentrations of unmeasured anions by proxy. Elevated concentrations of unmeasured anions like lactate, beta hydroxybutyrate, acetoacetate, PO, and SO, which rise with disease or intoxication, cause loss of HCO due to bicarbonate's activity as a buffer (without a concurrent increase in Cl−). Thus, finding a high anion gap should result in a search for conditions that lead to excesses of the unmeasured anions listed above.

A low anion gap is often due to hypoalbuminemia. Albumin is an anionic protein and its loss results in the retention of other negatively charged ions such as chloride and bicarbonate. As bicarbonate and chloride anions are used to calculate the anion gap, there is a subsequent decrease. The anion gap is sometimes reduced in multiple myeloma, where there is an increase in plasma IgG (paraproteinaemia).