Effect of moderate hypercholesterolemia on recognition of hypernatremia using indirect measurements of serum sodium and potassium concentrations.

Background: Na+ and K+ concentration measurements by ion-selective electrodes and flame photometry are affected by lipids influencing plasma water content.

Methods: In 166 sera with total cholesterol ranging from 1.19 to 23.3 mmol/L, and triglycerides ranging from 0.34 to 56.3 mmol/L, Na+ and K+ concentrations were assayed using flame photometry and direct ion-selective electrodes of the Vitros 950 analyzer. Linear regression analysis was used to examine inter-method difference as lipid content variable.

Results: In hypercholesterolemia and hypertriglyceridemia flame photometry yielded Na+ concentrations lower by 1.85% and 2.15% than ion-selective electrode measurements (p < 0.05). Na+ concentration inter-method difference correlated with total cholesterol and triglycerides. In bivariate analysis, inter-method difference significantly depended on total cholesterol (r2 = 0.09), whereas the impact of triglycerides was weak. In trivariate analysis, total cholesterol had the highest impact (r2 = 0.12), followed by Na+ concentration (r2 = 0.04), while the impact of triglycerides was insignificant. Recognition of hypernatremia significantly depended on total cholesterol (chi2 p = 0.0017); 27% of samples with hyperlipidemia (total cholesterol > 5.2 mmol/L, triglycerides > 2.5 mmol/L) classified as normonatremia by flame photometry presented hypernatremia by direct ion-selective electrode.

Conclusions: The lipid status of patients influences recognition of hypernatremia if assays are based on total plasma volume. No effect of lipids was observed on the recognition of hyperkalemia.