Discrepancies in Electrolyte Measurements by Direct and Indirect Ion Selective Electrodes due to Interferences by Proteins and Lipids.

We aim to report the simultaneous effect of different protein and lipid concentrations on sodium (Na ) and potassium (K ) measurement by direct and indirect ion selective electrodes (dISE and iISE) in patient samples.  Na and K were measured in 195 serum samples received in the laboratory using iISE by Roche Modular P800 autoanalyzer and using dISE by XI-921 ver. 6.0 Caretium electrolyte analyzer. Serum total protein (TP), cholesterol (Chol), and triglycerides (TG) were measured using conventional photometric methods on Roche Modular P800 autoanalyzer. Differences for each pair of results for Na (Diff_Na = [Na Na ]) and K (Diff_K = [K K ]) were calculated. Patient subgroups with high, normal, or low TP (< 5, 5-7.9, or ≥ 8 g/dL), Chol (< 150, 150-299, or ≥300 mg/dL), or TG (< 150, 150-299, or ≥300 mg/dL) were compared using analysis of variance. Note that 95% confidence interval of Diff_Na and Diff_K were calculated to see the number of samples showing clinically significant differences.  Diff_Na ( = 0.007) and Diff_K ( = 0.002) were found significant between samples with normal and high TP. However, effect of TG was not significant. Chol concentration affected Diff_Na significantly between low versus normal ( = 0.002), and high versus normal ( = 0.031) Chol groups. Diff_K was significant ( = 0.009) between low versus normal Chol. Clinically relevant disagreement of ≥|5| mmol/L for Na was observed in high percentage of samples including all subcategories; however, for K only 3.6% of the total samples showed disagreement of ≥ |0.5| mmol/L. A multivariate regression equation based on fit regression model was also derived.  Summarily, interchangeable use of electrolyte results from dISE and iISE is not advisable, especially in a setting of hyperproteinemia (≥8 g/dL) or hypercholesterolemia (≥300 mg/dL); more so for Na .