Anion Gap

The Anion Gap is a calculated value, not a direct measurement, derived from a patient's serum electrolyte results. It represents the difference between the primary measured cation (Sodium) and the primary measured anions (Chloride and Bicarbonate) in the plasma. The most common formula to calculate it is: Sodium - (Chloride + Bicarbonate). This calculation is clinically crucial for evaluating acid-base disorders, serving as the main tool to differentiate the causes of metabolic acidosis. A 'high anion gap metabolic acidosis' (elevated AG) indicates that the acidosis is caused by the accumulation of unmeasured acids (like ketoacids in diabetic ketoacidosis, lactate in lactic acidosis, or toxins like methanol and ethylene glycol). In contrast, a 'normal anion gap metabolic acidosis' (also called hyperchloremic acidosis) suggests that the acidosis is primarily due to the loss of bicarbonate, such as occurs in diarrhea or certain types of renal tubular acidosis.