Which method is susceptible to the solvent displacing effect, leading to falsely decreased electrolyte values?

• A. indirect ion selective electrodes
• B. direct ion selective electrodes
• C. spectrophotometric
• D. fluorescence

Answer: (A)

Solvent displacement occurs when the measurement assumes a constant water fraction in the sample, but high amounts of non-water solutes (like proteins or lipids) reduce the actual water content. Indirect ion-selective electrodes dilute the patient sample before measuring ions, so the electrolytes are assessed in a diluted solution whose water fraction is different from normal. If the sample has a reduced water fraction, the dilution causes the ion activity to appear lower, giving falsely decreased electrolyte values.

Direct ion-selective electrodes measure the undiluted sample, so they don’t rely on that diluted-water assumption and are not subject to this particular error. Spectrophotometric and fluorescence methods aren’t standard for routine electrolyte measurement in the same way and don’t depend on ion activity in a diluted aqueous phase, so they aren’t affected by solvent displacement in this context.