The anion gap is the difference between the number of cations versus anions. The anion gap is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions. The anion gap is a measurement of the difference-or gap-between the negatively charged and positively charged electrolytes. The anion gap is a value calculated from the results of multiple individual medical lab tests. The calculated anion gap is an artificial construct that merely reflects the fact that the concentration of the commonly measured cations exceeds the concentration.