Which statement is true about direct ion selective electrodes for electrolytes?

• A. Whole blood specimens are acceptable
• B. Elevated lipids cause falsely decreased results
• C. Elevated proteins cause falsely decreased results
• D. Elevated platelets cause falsely increased results

Answer: (A)

Direct ion-selective electrodes measure the activity of ions in an undiluted sample, so whole blood can be used without a dilution step. This approach keeps the sample’s natural water fraction intact, avoiding the distortions that occur when samples are diluted. Elevated lipids or proteins cause false decreases with the indirect (diluted) method because they reduce the fraction of plasma water available for the solutes, leading to pseudohyponatremia. Elevated platelets don’t create a predictable false increase in direct ISE measurements for these electrolytes. So, using whole blood as a specimen is true for direct ISE because it analyzes ion activity in the undiluted sample and bypasses dilution-related interferences.