Spurious Increases in the Anion Gap Due to Exposure of Serum to Air

To the Editor: The loss of water and carbon dioxide from serum samples, leading to overestimation of sodium, potassium, and chloride and underestimation of total carbon dioxide, has previously been demonstrated.^1,2 However, the effect of the above changes on the anion gap (calculated as [sodium + potassium] - [chloride + bicarbonate]) has not received adequate attention. Automated instruments, such as an AutoAnalyzer, introduce an appreciable error into the above determinations because the samples are not protected from exposure to room air. With the new generation of automated microinstruments, the magnitude of the above error is increased because of a greater.