The “anion gap” is a measure of the acidity of the blood resulting from a failure in the body’s ability to adjust for the waste-products of excessive metabolic activity. It is measured by 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 (8-16 mEq/L plasma), high, or low. A high anion gap indicates 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 and also in cancer.