Approach to hyponatremia: the value of fractional excretion of uric acid and phosphate

Abstract Introduction Differentiating syndrome of inappropriate antidiuretic hormone secretion (SIADH) from cerebral salt-wasting syndrome (CSWS) is a major challenge because of the identical clinical parameters that characterize their etiologies. Methods A 68-year-old woman presented to the emergency department with fever, chills, and headache. The patient had a history of multiple myeloma in active follow-up, with no recent chemotherapy treatment. During her hospital stay, the patient developed hypoosmolar hyponatremia. Results Initially, a diagnosis of CWS was made, but the torpid evolution of natremia led to reconsideration of the diagnosis. The use of fractional excretion of uric acid and phosphorus by the clinical laboratory established a definitive diagnosis, and clinical improvement was seen with appropriate therapeutic measures. Discussion This case report highlights the central role of the laboratory in the study of hyponatremia. The joint evaluation of biomarkers such as uric acid, fractional excretion of uric acid, phosphorus, and fractional excretion of phosphate allows for a more accurate differential diagnosis between SIADH and CSWS, overcoming the limitations of the volume approach and thus minimizing the risks associated with invasive or inconclusive therapeutic tests and delays in the implementation of therapeutic measures.