The main difference between a direct and indirect ISE is

The main difference between a direct and indirect ISE is

• A. Sample is diluted in the indirect method, not in the direct method
• B. The type of membrane that is used
• C. Direct ISEs use a reference electrode, whereas indirect ISEs do not
• D. Whole blood samples can be measured with the direct method and not with the indirect method

Answer: (A)

The key idea here is whether the sample is diluted before measurement. In a direct ISE, the electrode directly contacts an undiluted sample—often whole blood, serum, or plasma—so the ion activity is measured in the native sample matrix without a dilution step. In an indirect ISE, the sample is diluted with a buffer before measurement, so the electrode responds to the ion activity in the diluted aqueous phase rather than the original sample. This difference matters clinically because dilution assumes a constant water fraction in the sample; when the sample has unusually high protein or lipid content, the water fraction changes, leading to biased readings in indirect measurements (the electrolyte exclusion effect). Therefore, the dilution step in indirect ISEs is the defining difference from direct ISEs. The other statements don’t capture the main distinction: the membrane type is not inherently different between the methods; both use ion-selective membranes and reference electrodes in standard practice; and while direct ISEs can measure undiluted whole blood, indirect methods rely on dilution and are not described as measuring whole blood directly.

The key idea here is whether the sample is diluted before measurement. In a direct ISE, the electrode directly contacts an undiluted sample—often whole blood, serum, or plasma—so the ion activity is measured in the native sample matrix without a dilution step. In an indirect ISE, the sample is diluted with a buffer before measurement, so the electrode responds to the ion activity in the diluted aqueous phase rather than the original sample. This difference matters clinically because dilution assumes a constant water fraction in the sample; when the sample has unusually high protein or lipid content, the water fraction changes, leading to biased readings in indirect measurements (the electrolyte exclusion effect). Therefore, the dilution step in indirect ISEs is the defining difference from direct ISEs.

The other statements don’t capture the main distinction: the membrane type is not inherently different between the methods; both use ion-selective membranes and reference electrodes in standard practice; and while direct ISEs can measure undiluted whole blood, indirect methods rely on dilution and are not described as measuring whole blood directly.