<prompt> Answer this question truthfullyWhat is the plasma anion gap and how is it calculated?  <response>: <reason> The plasma anion gap represents unmeasured ions in serum and is equal to the concentration of sodium ions Na minus the sum of the concentrations of bicarbonate ions HCO3- and chloride ions Cl-. Therefore, the formula for calculating the plasma anion gap is Na - HCO3- Cl-. 