Can osmolality be normal despite an abnormal serum sodium concentration?
Yes! Serum sodium concentration is usually measured using indirect ion-selective electrodes with specimen dilution and depends on the assumption that water comprises approximately 93% of plasma. The presence of high concentrations of plasma lipid or protein, however, will reduce the aqueous contribution to plasma volume. As a result, the measured serum sodium concentration will be falsely low. This condition is known as pseudohyponatremia. Ultracentrifugation and separation of the lipid layer can correct for lab artifact due to hyperlipidemia. Direct ion-selective electrodes are not confounded by hyperlipidemia or hyperproteinemia, but only about a third of chemical analyzers use this technique. Blood gas laboratories use ion-selective electrodes and thus are not susceptible to this artifact.
