MPAS Common Laboratory Tests: Blood Practice Test

When pseudohyponatremia is suspected due to lipid abnormalities, the true sodium level is best confirmed by measuring with a direct ion-selective electrode. This method analyzes undiluted plasma, so it measures the sodium activity in the actual plasma water without relying on the assumed water fraction. Indirect ion-selective electrode measurements involve diluting the sample and assume a normal plasma water fraction; with excess lipids or proteins, the water fraction is reduced, leading to a falsely low reading that can mask the true sodium level. Spectrophotometric assays aren’t used for routine serum sodium, and while potentiometric methods describe the principle behind ion-selective electrodes, the key distinction here is avoiding dilution, which direct ISE provides.