The anion gap[1][2] (AG or AGAP) is a value calculated from the results of multiple individual medical lab tests.
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.
)[citation needed] The cations calcium (Ca2+) and magnesium (Mg2+) are also commonly measured, but they aren't used to calculate the anion gap.
The anion gap varies in response to changes in the concentrations of the above-mentioned serum components that contribute to the acid-base balance.
Laboratory errors need to be ruled out whenever anion gap calculations lead to results that do not fit the clinical picture.
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.
Elevated concentrations of unmeasured anions like lactate, beta-hydroxybutyrate, acetoacetate, PO3−4, and SO2−4, which rise with disease or intoxication, cause loss of HCO−3 due to bicarbonate's activity as a buffer (without a concurrent increase in Cl−).
Raised levels of acid bind to bicarbonate to form carbon dioxide through the Henderson-Hasselbalch equation resulting in metabolic acidosis.
[citation needed] Note: a useful mnemonic to remember this is FUSEDCARS – fistula (pancreatic), uretero-enterostomy, saline administration, endocrine (hyperparathyroidism), diarrhea, carbonic anhydrase inhibitors (acetazolamide), ammonium chloride, renal tubular acidosis, spironolactone.
[citation needed] The anion gap is sometimes reduced in multiple myeloma, where there is an increase in plasma IgG (paraproteinaemia).
[9][16][17] Common conditions that reduce serum albumin in the clinical setting are hemorrhage, nephrotic syndrome, intestinal obstruction and liver cirrhosis.
In the largest study published to date, featuring over 12,000 data sets, Figge, Bellomo and Egi[18] demonstrated that the anion gap, when used to detect critical levels of lactate (greater than 4 mEq/L), exhibited a sensitivity of only 70.4%.
Therefore, it is important to correct the calculated value of the anion gap for the concentration of albumin, particularly in critically ill patients.
[18][19][20] Corrections can be made for the albumin concentration using the Figge-Jabor-Kazda-Fencl equation to give an accurate anion gap calculation as exemplified below.