Certain conditions that interfere with laboratory tests of serum sodium concentration such as extraordinarily high blood levels of lipid (hyperlipidemia/hypertriglyceridemia) or protein (hyperparaproteinemia) may lead to an erroneously low measurement of sodium. This is called pseudohyponatremia, and can occur when laboratories use the flame-photometric and indirect (but not direct) ion-selective electrode assays. This is distinct from a true dilutional hyponatremia that can be caused by an osmotic shift of water from cells to the bloodstream after large infusions of mannitol or intravenous immunoglobulin.
^Nguyen MK, Ornekian V, Butch AW, Kurtz I (May 2007). "A new method for determining plasma water content: application in pseudohyponatremia". Am. J. Physiol. Renal Physiol.292 (5): F1652–6. doi:10.1152/ajprenal.00493.2006. PMID17299138.