pK1' and bicarbonate concentration in plasma

Area of Science:

• Biochemistry
• Clinical Chemistry
• Medical Diagnostics

Background:

• Accurate determination of blood gas parameters is crucial for clinical decision-making.
• The pK1' constant is a key factor in calculating plasma bicarbonate concentrations ([HCO3]p).
• Existing methods may not fully account for the variability of pK1' in different patient populations.

Purpose of the Study:

• To investigate the variability of the pK1' constant in human plasma.
• To assess the impact of this variability on bicarbonate concentration calculations.
• To evaluate the accuracy of blood-gas analyzers in diverse clinical scenarios.

Main Methods:

• pK1' values were measured at 37°C in 443 fresh plasma samples from healthy, seriously ill, and hyponatremic patients.
• Plasma samples were tonometered, and pCO2 values were calculated.
• Replicate plasma dilutions were analyzed with controlled ionic strength and varying bicarbonate and sodium concentrations.

Main Results:

• Significant variations in pK1' values were observed, exceeding 0.06 in healthy and critically ill individuals.
• Bicarbonate concentrations calculated using a standard pK1' of 6.1 could exhibit errors of up to +/- 60%.
• pK1' decreased with increasing [HCO3]p at constant ionic strength and was higher at elevated pCO2 levels.

Conclusions:

• The pK1' constant is not a fixed value and exhibits considerable variability in human plasma.
• Standard pK1' values used in blood-gas analyzers can lead to significant inaccuracies in bicarbonate measurements.
• Clinical interpretations based on calculated bicarbonate levels may require re-evaluation due to pK1' variability.