Spurious test results as a diagnostic clue - pseudohyponatremia revealing multiple myeloma

To describe a case of a pseudohyponatremia secondary to hyperproteinemia in multiple myeloma and to emphasize the importance of analytical verification and laboratory-clinical collaboration to prevent diagnostic errors.

A 55 year-old male presented with severe hyponatremia (119 mmol/L) but paradoxically elevated serum osmolarity, suggesting that the hyponatremia might represent a laboratory artifact. Repeat serum sodium measurement by direct potentiometry with ion-specific electrodes yielded a normal value of 137 mmol/L, confirming artifactual hyponatremia. Additional tests revealed marked hyperproteinemia due to an IgG-kappa monoclonal component, leading to the diagnosis of multiple myeloma.

This case highlights the importance of artifacts in laboratory measurement, in this case recognizing analytical interferences when discordant hyponatremia is found. Confirmatory testing by direct potentiometry ion-specific electrodes prevents misdiagnosis and inappropriate therapy. Early identification of pseudohyponatremia further emphasizes the crucial role of the laboratory in ensuring accurate interpretation and optimizing patient management.