The difference between the measured cations sodium (Na+) and potassium (K+) and the measured anions chloride (CP) and bicarbonate (HC03_). In accordance with the principle of electroneutrality, in any body fluid the number of net positive charges contributed by cations must equal the number of net negative charges contributed by anions. The apparent difference is accounted for by the unmeasured anions present (the anion gap); these include lactate, sulfates, phosphates, proteins, ketones, and other organic acids. In general, an anion gap of 8 to 18 mmol/L is normal. An increased value is present in some forms of metabolic acidosis.