Pseudohyponatremia is commonly caused by which condition?

• A. Hyperlipidemia
• B. Hyperproteinemia
• C. Hyperglycemia
• D. Sorbitol/mannitol

Answer: (A)

Pseudohyponatremia happens when a lab artifact makes the sodium appear low despite the actual plasma water sodium being normal. The classic trigger is marked increases in non-aqueous components of blood, especially lipids or proteins. In indirect measurements, the sample is diluted, and a larger non-water fraction (from hyperlipidemia or hyperproteinemia) reduces the proportion of water in serum. Since sodium sits in the aqueous phase, its reported concentration falls even though the true sodium content in plasma water hasn’t changed. That’s why hyperlipidemia is the most common cause.

Hyperproteinemia can cause the same artifact, though it’s less typically the focus in exams. The key contrast is with hyperglycemia, which lowers measured sodium through a real osmotic shift of water (true hyponatremia) and raises serum osmolality, not pseudohyponatremia.

If you suspect pseudohyponatremia, check serum osmolality (it will be normal) and consider using a direct ISE measurement (which isn’t affected by the non-aqueous fraction) to confirm. Treat the underlying lipid or protein abnormality rather than the sodium value itself.