In a patient with multiple myeloma, indirect ISE is more likely to give erroneous Na results than direct ISE because of which scenario?

In a patient with multiple myeloma, indirect ISE is more likely to give erroneous Na results than direct ISE because of which scenario?

• A. Excess protein binds Na in indirect ISE
• B. Na falsely increased by indirect ISE
• C. Na falsely decreased by indirect ISE
• D. Excess protein reacts with diluent in indirect ISE

Answer: (C)

The key idea is how indirect ISE handles dilution. Indirect ISE dilutes the plasma sample and assumes a constant water fraction (about 93%) in which sodium resides. In multiple myeloma, there is a large amount of nonaqueous protein in the plasma, which lowers the actual water fraction. When the sample is diluted, there is less water available for the sodium, so the measured sodium concentration in the diluted sample appears lower than the true value. This creates pseudohyponatremia, an erroneous low sodium result that indirect ISE is prone to in these conditions. Direct ISE, measuring Na in undiluted whole blood, is not affected by the reduced water fraction, so it yields a more accurate value.

The key idea is how indirect ISE handles dilution. Indirect ISE dilutes the plasma sample and assumes a constant water fraction (about 93%) in which sodium resides. In multiple myeloma, there is a large amount of nonaqueous protein in the plasma, which lowers the actual water fraction. When the sample is diluted, there is less water available for the sodium, so the measured sodium concentration in the diluted sample appears lower than the true value. This creates pseudohyponatremia, an erroneous low sodium result that indirect ISE is prone to in these conditions. Direct ISE, measuring Na in undiluted whole blood, is not affected by the reduced water fraction, so it yields a more accurate value.