Determination of plasma bicarbonate of neonates in intensive care.

Most modern blood-gas analyzers are programmed to use the Henderson-Hasselbalch equation to calculate a value for plasma bicarbonate. It has been suggested, however, that among acutely ill patients, including newborns, these calculated values may be at variance with measured total CO2. To assess the clinical significance of such errors, we compared calculated bicarbonate with measured total CO2 in 79 blood samples from 40 babies in intensive care. The calculated bicarbonate values consistently exceeded the measured values by about 1.5 mmol/L. Of the errors, 94% were within the range -10% to +20%. When the systematic bias was removed, calculated and measured bicarbonate values agreed within +/- 3.30 mmol/L in 95% of cases. Because calculated values can be obtained much more quickly and frequently than laboratory measurements, we believe that these limits are clinically acceptable.