If serum sodium is decreased with normal osmolality, which measurement should be used to determine sodium?

If serum sodium is decreased with normal osmolality, which measurement should be used to determine sodium?

• A. Check for glucose
• B. Measure sodium using direct ISE
• C. Sample is hemolyzed
• D. Measure sodium using indirect ISE

Answer: (B)

When serum sodium appears low but the osmolality is normal, the result may be a false reading caused by the measurement method reacting to the nonaqueous portion of plasma (lipids or proteins). This is pseudohyponatremia, where the actual sodium concentration in the aqueous phase is not truly low. Direct ion-selective electrode (ISE) measurement analyzes undiluted serum or plasma, so it measures sodium activity in the actual aqueous fraction without being distorted by the solids fraction. This makes it the most reliable way to determine true sodium when you suspect pseudohyponatremia. Indirect ISE, on the other hand, dilutes the sample. If there is an elevated lipid or protein level, the proportion of nonaqueous content is larger, so the diluted sample yields a falsely low sodium value. That’s why indirect ISE can mislead in this scenario. Checking glucose isn’t the correction here, since normal osmolality argues against hyperglycemia-driven hyponatremia, and a hemolyzed sample would introduce other artifacts but doesn’t address the underlying measurement accuracy for sodium in this context.

When serum sodium appears low but the osmolality is normal, the result may be a false reading caused by the measurement method reacting to the nonaqueous portion of plasma (lipids or proteins). This is pseudohyponatremia, where the actual sodium concentration in the aqueous phase is not truly low.

Direct ion-selective electrode (ISE) measurement analyzes undiluted serum or plasma, so it measures sodium activity in the actual aqueous fraction without being distorted by the solids fraction. This makes it the most reliable way to determine true sodium when you suspect pseudohyponatremia.

Indirect ISE, on the other hand, dilutes the sample. If there is an elevated lipid or protein level, the proportion of nonaqueous content is larger, so the diluted sample yields a falsely low sodium value. That’s why indirect ISE can mislead in this scenario.

Checking glucose isn’t the correction here, since normal osmolality argues against hyperglycemia-driven hyponatremia, and a hemolyzed sample would introduce other artifacts but doesn’t address the underlying measurement accuracy for sodium in this context.