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

Abstract

Objectives 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. Materials and Methods 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+ dISE–Na+ iISE]) and K+ (Diff_K+ = [K+ dISE–K+ iISE]) 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. Results Diff_Na+ (p = 0.007) and Diff_K+ (p = 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 (p = 0.002), and high versus normal (p = 0.031) Chol groups. Diff_K+ was significant (p = 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. Conclusion 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+.

Comparison of selected serum biochemistry measurements between the Nova Prime Plus VET, Nova pHOx Ultra, and Beckman Coulter AU680 analyzers in dogs

“…The Prime Plus VET analyzer met all manufacturer quality control targets during the study, indicating that the increased variability was most likely the result of matrix effects. Interestingly, we could identify only two studies that compared measurements of serum or plasma electrolytes using direct and indirect ISE methods, and both studies reported a similar degree of variability in measuring Na concentration 15,16 to that observed in our study, suggesting the presence of a matrix effect. The increased variability in our study was not due to abnormal total protein concentrations, but because linear regression did not identify a significant relationship between the difference in serum Cl, K, and Na concentrations measured by the Prime Plus VET and Beckman chemistry analyzers and serum total protein concentration.…”

Comparison of lipemia interference created with native lipemic material and intravenous lipid emulsion in emergency laboratory tests

“…In cases of hyperlipidemia and hyperproteinemia, Na, K and Cl are measured as falsely low by the indirect method, whereas the direct method is unaffected (2). As in the study of Chopra et al, Na and K values measured by the direct method were higher than the autoanalyzer results (36). Contrary to the information in distinction was not fully observed for some other tests.…”

Agreement of Potassium, Sodium, Glucose, and Hemoglobin Measured by Blood Gas Analyzer With Dry Chemistry Analyzer and Complete Blood Count Analyzer: A Two-Center Retrospective Analysis

“…The indirect ISE measurement technique uses a predetermined volume of ionic solution to dilute the plasma sample, usually adjusting the assay concentration by a fixed factor of 0.93 to account for the effect of plasma composition. Abnormal concentrations of proteins or other Non-aqueous components can introduce errors by altering the plasma water content (18,19). An estimated 1/4 of indirect ISE ICU plasma sodium measurements differ from the corresponding direct ISE values by at least 4 mmol/L, the dominant factor being indirect ISE overestimation associated with hypoproteinemia (20).…”