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There is increasing evidence of variability in pK1', the practical dissociation coefficient used in the Henderson-Hasselbalch equation to calculate arterial bicarbonate from measurements of arterial pH and pco2. The case presented here illustrates not only potential technical artifacts in arterial blood sampling, which can confuse, but also irreconcilable differences in the values of calculated arterial bicarbonate vs measured arterial and venous total carbon dioxide (carbon dioxide content). Measurements of total carbon dioxide in arterial blood will resolve such conflicts, particularly for acutely ill patients, and will reflect the correct bicarbonate measurements for use in therapeutic decisions.