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.