Effects of pK variability on bicarbonate balance evaluation in dialysis patients

Bicarbonate balance is usually calculated from the Henderson-Hasselbalch equation under the assumption that pK is constant. To evaluate the pK variability and its effects on bicarbonate computed with autoanalyzers in dialysis patients, we studied 44 patients on maintenance hemodialysis. The pH, PCO2, total CO2, and pK were determined in each patient before and after the dialysis session. The mean total CO2 calculated (22.5 +/- 2.7 meq/L) from pH and PCO2 values was significantly higher (p less than 0.001) than that directly measured (20.4 +/- 3.0 meq/L) with total CO2 analyzer. The mean (+/- SD) pK value was 6.14 +/- 0.06 (range 6.05-6.28). The percentage error in computed bicarbonate due to pK variations from the traditional pK value of 6.1 ranged between -11% and 52%. The pK value changed during dialysis in the majority of our patients, thereby confirming that pK consistency does indeed vary. Thus, investigation of acid-base balance based on pH and PCO2 determination may lead to erroneous results determined by pK abnormalities.