Anion gap: A measurement of the interval between the sum of routinely measured cations minus the sum of the routinely measured anions in the blood. The anion gap = (Na+ + K+) - (Cl- + HCO3-) where Na-is sodium, K+ is potassium, Cl-is chloride, and HCO3- is bicarbonate.The anion gap can be normal, high, or low.A high anion gap indicated metabolic acidosis, the increased acidity of the blood due to metabolic processes. A low anion gap is relatively rare but may occur from the presence of abnormal positively charged proteins, as in multiple myeloma.he anion gap = (Na+ + K+) - (Cl- + HCO3-) where Na-is sodium, K+ is potassium, Cl-is chloride, and HCO3- is bicarbonate. The anion gap can be normal, high, or low. A high anion gap indicated metabolic acidosis, the increased acidity of the blood due to metabolic processes.