The reliability of calculated bicarbonate in clinical practice

Current clinical laboratory methods utilize the Henderson-Hasselbalch equation to calculate plasma bicarbonate from measured pH and pCO2. This practice assumes that the apparent first dissociation constant for carbon dioxide in serum, pK'1, is invariable in clinical situations. This assumption has been questioned recently. Our study examined arterial blood samples from 50 acutely ill patients who were routinely sent for blood gas analysis. The pH and pCO2 were measured on a blood gas analyzer and the total CO2 was determined on the same blood sample using a microgasometer. The calculated pK'1 from these parameters was 6.10 +/- 0.018 (m +/- SD) with a range of 6.06 to 6.15. This amount of variability could be explained by the error in the methods. The correlation for calculated to measured total CO2 was y = 094x + 1.81, r = 0.98, p less than 0.001. These findings indicate that pK'1 is functionally fixed in clinical practice and that the methods used to calculate serum bicarbonate are acceptable.