Sodium, Potassium, and Chloride Measurements by an Indirect Ion- Selective Electrode Measuring System To the Editor:

We used indirect ion-selective elec-trode (ISE) methods (Roche/Hitachi Modular) to investigate the effect of hyperlipidemia (cholesterol plus triglycerides) on measurements of sodium, potassium, and chloride and the ability of published formulas to correct for the decrease in measured Na, K, and Cl. As recommended by Kroll (1), we did not use enriched samples but instead used 21 patient samples (lithium-heparin plasma and serum) with triglyceride concen-trations15 mmol/L, total lipid con-centrations (cholesterol plus triglyc-erides; Roche/Hitachi Modular) 20 mmol/L, a lipemic index (Modular) 3, and a total protein concentration within the reference interval (62–83 g/L). We analyzed samples before and after ultracentrifugation (Air-fuge; Beckman Coulter; 15 min at 107 000g) by indirect ISE potentiom-etry (Modular) and direct ISE (Rap-idlab 865; Bayer). The ultracentrifuged indirect ISE result (y) and nonultracentrifuged di-rect ISE result (z) were subtracted from the nonultracentrifuged indi-rect ISE result (x; see Fig. 1 in the Data Supplement that appears with the online version of this letter a