Sodium measurement: effects of differing sampling and analytical methods

Area of Science:

• Clinical Chemistry
• Biomedical Analysis

Background:

• Pediatric patients on intravenous fluids require frequent biochemical monitoring, necessitating high-precision sodium analysis.
• Current practice sometimes uses capillary blood (direct ion-selective electrode) and venous plasma (indirect ion-selective electrode) interchangeably.
• Variability in sample collection and measurement methods may impact clinical decision-making.

Purpose of the Study:

• To assess if capillary blood (direct ISE) and venous plasma (indirect ISE) sodium measurements are interchangeable for clinical monitoring.
• To determine the impact of combined sampling and analytical variability on sodium measurement accuracy.

Main Methods:

• Analyzed sodium levels in 57 adults using both capillary and venous blood samples.
• Employed direct ion-selective electrode (ISE) for capillary blood and indirect ISE for venous plasma.
• Compared sodium measurements obtained from different sample types and ISE methods.

Main Results:

• Significant scatter and poor correlation (r=0.36) were observed between capillary (direct ISE) and venous plasma (indirect ISE) sodium measurements.
• Substantial biases were detected when comparing capillary vs. venous sodium (P < 0.001) and direct vs. indirect ISE methods (P < 0.001).
• The standard deviation of differences was 2.7 mmol/L, with limits of agreement ranging from -10.6 to 10.6 mmol/L.

Conclusions:

• Venous plasma analyzed by indirect ISE and capillary blood analyzed by direct ISE are not interchangeable for detecting small sodium changes.
• Using single, consistent methods for both sampling and analysis is crucial to prevent misinterpretation of sodium monitoring results.
• Standardized protocols are essential for accurate pediatric biochemical monitoring.