Once metabolic acidosis is apparent, the problem becomes one of identifying the cause. Calculation of the anion gap may be helpful. The anion gap is the difference between the major cations (sodium, or sodium plus potassium) and the major anions (chloride and bicarbonate). The anion gap formula is: AG = Na – (C1 + HCO–3). If the anion gap is increased, and especially when it is more than 10 mEq/L above the upper limit of the reference range, excess organic acids or acidic foreign substances should be suspected. Conditions in which these may appear include diabetic ketoacidosis, ethyl alcohol-induced ketoacidosis, renal failure, lactic acidosis, salicylate overdose, and methanol or ethylene glycol poisoning. The value most often listed for normal anion gap is 8-16 mEq/L (mmol/L). However, there is some disagreement in the literature whether to use a range of 8-12 or 8-16 mEq/L for a normal anion gap. Some investigators use the sum of sodium plus potassium in the equation rather than sodium alone. Although one would expect this to decrease the normal anion gap, the reference values reported in the literature are the same or even greater than those for the formula using sodium alone, with some listing a range of 8-16 mEq/L and others 8-20 (values in the literature can be found extending from 7-25 mEq/L). Anion gap reference ranges established on hospitalized patients tend to be higher than those established on outpatients. A collection tube filled to less than one third of tube capacity can result in a falsely decreased bicarbonate and falsely increased anion gap.

A decreased anion gap has been associated with multiple myeloma. However, one report indicates that most calculated anion gaps that are decreased result from laboratory error in test results included in the equation, with hypoalbuminemia and hyponatremia the next most common associated findings.