plasma anion gap CMO:0000066 The plasma anion gap is a measurement of the difference-or gap-between the negatively charged and positively charged electrolytes in plasma, i.e. blood from which cells and cellular components have been removed. If the anion gap is either too high or too low, it may be a sign of a disorder in the lungs, kidneys, or other organ systems. VT:0001765 clinical_measurement_ontology anion gap