Which statement best describes direct ISE measurement of electrolytes in blood?

Which statement best describes direct ISE measurement of electrolytes in blood?

• A. The sample is diluted prior to measurement
• B. The measurement is performed on undiluted whole blood
• C. It cannot measure sodium in serum
• D. It uses a flame to calibrate

Answer: (B)

Direct ISE measures electrolytes by sensing ion activity in the sample without diluting it. In practice, this means the measurement is performed on an undiluted sample, such as whole blood or plasma, so the electrode responds to the ions present in the sample’s natural aqueous phase. This contrasts with indirect ISE, which dilutes the sample before measurement and can be affected by the sample’s protein or lipid content. Why this fits the statement: the key feature of direct ISE is the lack of dilution, giving readings from the undiluted sample. The other options don’t fit because diluting the sample describes indirect ISE, sodium can be measured in serum with direct ISE, and calibration by flame is not part of ISE methods.

Direct ISE measures electrolytes by sensing ion activity in the sample without diluting it. In practice, this means the measurement is performed on an undiluted sample, such as whole blood or plasma, so the electrode responds to the ions present in the sample’s natural aqueous phase. This contrasts with indirect ISE, which dilutes the sample before measurement and can be affected by the sample’s protein or lipid content.

Why this fits the statement: the key feature of direct ISE is the lack of dilution, giving readings from the undiluted sample. The other options don’t fit because diluting the sample describes indirect ISE, sodium can be measured in serum with direct ISE, and calibration by flame is not part of ISE methods.