Nephrology

Answer 5 Serum osmolality. This is a case of pseudohyponatremia related to laboratory technique. If the nonaqueous phase of plasma is expanded, such as with high triglyceride levels, assays that employ plasma dilution while measuring serum sodium levels may report an abnormally low value. However, these lipids and proteins do not contribute to serum osmolality, which remains in large part a function of serum sodium (2 × plasma Na + [glucose/18] + [blood urea nitrogen/2.8]). Therefore, pseudohyponatremia is diagnosed when serum osmolality is normal. Ion-selective electrodes for sodium will also reveal a normal sodium level. Pseudohyponatremia can also be seen with multiple myeloma and the production of paraproteins. Urine osmolality, urine sodium, and serum uric acid can vary and have no role in diagnosing pseudohyponatremia. Click here to return to the questions

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