A severely lipemic specimen has sodium 121 mmol/L by an indirect ion-selective electrode, sodium 138 mmol/L by a direct ion-selective electrode, and a measured serum osmolality within the reference interval. What best explains the discrepancy?
1True hypotonic hyponatremia from excess water retention diluting all body fluids, including plasma
2Sodium contamination of the direct electrode from a heparinized collection tube during sampling
3Water loss during specimen transport concentrating the sample for the indirect method measurement
4Pseudohyponatremia caused by the electrolyte exclusion effect in the diluted indirect method✓ 정답
해설
Indirect ISE methods dilute the specimen and assume a normal plasma water fraction. Markedly increased lipids reduce that fraction and can produce an artifactually low sodium result, while direct ISE avoids the dilution-related electrolyte exclusion effect.