Serum or plasma bicarbonate results are often the first step in assessing for an acid-base disorder. Bicarbonate levels can either be estimated by measuring “total CO2,” which includes bicarbonate and dissolved CO2, using enzymatic assays or calculated based on the Henderson-Hasselbalch equation using pH and PCO2 measured by blood gas analyzers. The average difference between measured and calculated bicarbonate has been reported to be within 3 mmol/L.1 However, all biochemical tests can be influenced by interfering chemicals that react with reagents or modify signals, which can lead to erroneous values.