Serum Anion Gap Medical Calculator

• Anion Gap (without K) = Na - (Cl + HCO3)
• Anion Gap (with K) = (Na + K) - (Cl + HCO3)
• Delta Gap (without K) = [Na - (Cl + HCO3)] - 12

• 8 - 12 mEq/L or mmol/L

Increased Anion Gap:

Generally considered to be >12 ± 2 mEq/L or mmol/L and reflect the anions that are not included in the formula (albumin, phosphate, sulfate).  Therefore, if the anion gap is > 12 mEq/L this would suggest that additional anions are also present when they should not be.  These additional anions that can cause  metabolic acidosis where bicarbonate (HCO3) is consumed by negatively charged particles include: ethylene glycol (due to the formation of oxalic acid), ketones, lactate, metabolites of methanol (i.e., formic acid), ketoacids (acetoacetate & β-hydroxybutyrate), and salicylates.  In addition, increased albumin (hyperalbuminemia) and uremia can increase anions (or negative charges present) and hypocalcemia or hypomagnesemia can decrease the cations (positive charges present).  Alcoholics can also develop an anion gap from lactic acidosis (due to a greater amount of pyruvate being converted to lactate from the excess NADH generated in alcohol metabolism).

Decreased Anion Gap:

Generally considered to be <8 mEq/L or mmol/L. This is not usually associated with metabolic acidosis but can be seen in the presence of decreased anions such as albumin levels or in the presence of increased cations such as hypercalcemia, hypermagnesemia, hypergammaglobulinemia or lithium intoxication.

Normal Anion Gap Acidosis:

Usually due to a loss of HCO3 or decreased ability to make or reclaim HCO3 in the kidney. As such, Cl anions are increased to counterbalance the loss of HCO3. This increase in Cl anions helps to maintain electrical neutrality. Causes can include: diarrhea (due to loss of HCO3 in the stool), cholestyramine, and type I, II, and IV distal renal tubular acidosis.

Delta Gap:

This is simply the difference in the calculated anion gap (not including potassium) and a normal anion gap of 12.  The delta gap is useful in assessing the bicarbonate levels and thus is considered a HCO3 equivalent.  As such, for unit increase in the anion gap, the HCO3 should be lowered by 1. Therefore, if the delta gap value is added to the measured HCO3, then that result should be in the normal range for HCO3.  If there is an elevation noted in the adjusted HCO3, then the presence of a metabolic alkalosis may also be present thereby indicating the presence of a mixed acid-base disorder.