The anion gap is a calculated value derived from the concentrations of major serum electrolytes, primarily used to identify the cause of metabolic acidosis. It is determined by measuring the difference between the measured cations, sodium (Na+), and the measured anions, chloride (Cl-) and bicarbonate (HCO3-). The formula is: Anion Gap = [Na+] - ([Cl-] + [HCO3-]) Typical reference values for the anion gap range from 8 to 16 mmol/L. A raised anion gap is suggestive of the presence of unmeasured anions, often associated with metabolic acidosis due to accumulation of acids in the body.
