abnormal plasma anion gap decreased plasma anion gap reduced plasma anion gap MPheno.ontology less than the normal 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; low anion gap is relatively rare but may occur from the presence of abnormal positively charged proteins, as in multiple myeloma MP:0005626 low anion gap