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

OBJECTIVES: 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. CASE PRESENTATION: 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. CONCLUSIONS: 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.