A comparative study of serum electrolytes measured using flame photometry, direct ion-selective electrode method and indirect ion-selective electrode method

Abstract Background: Earlier studies showed differences in the values of serum electrolytes obtained by using different methods. In the present study three methods used for the measurement of serum electrolytes namely flame photometry (FP), direct ion-selective electrode method (ISE), and indirect ISE were compared to evaluate whether the values obtained by the methods are similar and interchangeable. Methods: Serum sodium and potassium levels were analyzed in 40 samples using the three methods. The United States Clinical Laboratory Improvement Amendments (US CLIA) 1988 guideline of acceptability of a difference of 0.5 mmol/L in potassium level and 4.0 mmol/L in sodium level compared to target values was considered as quality goal. Results: Significant difference was observed between paired values of all three methods of both electrolytes. The difference in sodium levels between FP and both ISE methods exceeded acceptable level while the difference in sodium levels between direct and indirect ISE methods is within the range given by US CLIA 88 guidelines. Serum potassium levels obtained from paired comparisons of the three methods was <0.5 mmol/L and hence was acceptable as per US CLIA 1988 guidelines. Significant positive correlation was found between electrolyte levels obtained by the three methods. There was a good agreement between direct ISE and indirect ISE methods for measurement of sodium. For potassium, there was a good agreement between paired values of all the three methods. Conclusions: Findings of present study indicate the importance of determining concordance between electrolyte values obtained using different methods. The differences that exist should be considered while interpreting the electrolyte levels.