An anion gap refers to the difference between the concentration of positively charged ions, known as cations and negatively charged ions, known as anions, in a liquid. The anion gap is often measured as part of medical tests for a variety of conditions.
The anion gap is often calculated from a patient's urine sample. The positive ions that are typically measured are potassium and sodium, although potassium is often ignored since it's usually much smaller than potassium. The anions measured in a typical test are chloride and bicarbonate. A high anion gap may demonstrate that a patient is in acidosis, a common state in diabetics. High anion gaps are far more common than low gaps.