Discrepancies between sodium concentrations measured by the Kodak Ektachem 700 and by dilutional and direct ion-selective electrode analyzers.

We have identified rare (approximately 0.2% of all samples), but clinically significant, discrepancies between serum or plasma sodium concentrations measured with the Kodak Ektachem 700's direct ion-selective electrode (ISE) method and concentrations measured with two other analyzers: the Beckman Synchron CX3's dilutional ISE instrument and the Radiometer KNA2 instrument for sodium-potassium analysis by the direct ISE method. The differences do not appear to be related to any previously identified sources of discrepancy, such as variations in triglycerides, bicarbonate, total protein, albumin, or gamma-globulin, the presence of paraproteins, or interference by benzalkonium chloride from heparinized catheters. They occurred despite the use of Gen 04 reference fluid on the Ektachem. We could not identify any drug or family of drugs that the patients had taken in common and that might influence the results. Until this problem is resolved, Ektachem users should be aware of the potential for discrepancies of > 6 mmol/L in measurements of sodium concentrations.