Determination of pKA of nonvolatile weak acids in plasma of healthy volunteers and critically ill patients.

Background: The dissociation constant of nonvolatile weak acids in plasma (K_A), expressed as pK_A, is essential for electroneutrality-based acid-base analysis. To date, its normal value in human plasma has been determined in only one study involving eight healthy volunteers. We hypothesized that pK_A would differ in ICU patients, whose plasma protein composition is altered by disease and medication, and that changes in protein charge-rather than undetected strong acids-could account for the unexplained anions observed in sepsis.

Methods: Using CO₂ tonometry, we determined pK_A and total weak nonvolatile acids (A_TOT) in plasma from 30 healthy volunteers and two ICU cohorts (27 postoperative and 30 septic patients). Additionally, we calculated the strong ion gap in plasma and protein-free serum filtrates from 10 healthy volunteers and 20 septic patients.

Results: In healthy volunteers, pK_A was 7.55 ± 0.16 (K_A = 2.8 × 10⁻⁸) and A_TOT was 15.9 ± 3.0 mmol/L (0.222 ± 0.043 mmol/g of TP). In postoperative and septic patients, A_TOT was significantly reduced (10.1 ± 5.4 and 11.9 ± 4.0 mmol/L, p < 0.001), but pK_A and A_TOT/TP remained unchanged, yielding an average pK_A of 7.55 ± 0.35 (K_A = 2.8 × 10⁻⁸) and A_TOT/TP of 0.230 ± 0.097 mmol/g. We found elevated strong ion gap in both plasma and protein-free filtrates of septic patients, which confirms the presence of unmeasured low-molecular-weight anions.

Conclusion: Our findings confirm stable pK_A and A_TOT/TP values in human plasma in both health and disease, supporting the Staempfli-Constable model for clinical acid-base diagnostics. Unexplained anions in sepsis are attributed to low molecular weight strong ions rather than alterations in plasma protein dissociation.