Case of the Month #44 High Anion Gap Metabolic Acidosis

Published 12/10/2023

Why are some values incalculable on the blood gas?

Blood gas analysers measure some analytes directly. These include pH, pCO2, pO2, Na+, K+, Ca2+, Cl-, glucose, lactate, Hb, SO2.

Other measurements are derived from directly measured analytes. These include HCO3- and the base excess (BE).

Measured analytes have a ‘measurable range’ which allow reliable values to be reported. The pH (<6.80) was outside of the measurable range, meaning an absolute value could not be placed.

Given the formula to derive the BE of blood requires a pH value and the formula to derive a HCO3- requires a BE value, neither can be calculated.