Multiple Myeloma Presenting as Pseudohyponatremia

Introduction: Multiple myeloma (MM) is a malignant plasma cell disorder characterized by CRAB features: hypercalcemia, Renal failure, Anemia, and Bone lesions. Occasionally, patients may present with atypical biochemical abnormalities such as pseudohyponatremia due to paraproteinemia. Recognizing this lab artifact is critical to avoid misdiagnosis and inappropriate treatment. Case Description: A 63 year old Jamaican woman, recently immigrated to the U.S.,presented with 3 months of progressive weight loss (25 lbs.), anorexia, intermittent nausea, and an episode of black tarry stool. She denied dysuria or frank hematuria but noted mild urinary discomfort. Initial workup revealed: Hb 5.5 g/dL (NC, NC), s. Cr 8.3 mg/dL and BUN 90 mg/dL,S. Na 125 mmol/L with elevated total protein (14.3 g/dL) and hypoalbuminemia,Hypercalcemia (12.4 mg/dL),Positive stool guaiac, but negative EGD. CT abdomen / pelvis showed diffuse osseous lytic lesions. Further labs showed elevated serum free kappa light chains (870.1 mg/L) with a kappa/lambda ratio >800, supporting a diagnosis of IgG-kappa multiple myeloma. Bone marrow biopsy confirmed the diagnosis. She was started on CyBorD chemotherapy (Cyclophosphamide, Bortezomib, Dexamethasone) and showed improvement in renal function and symptoms.The patient’s initial presentation raised concern for GI bleeding and acute kidney injury. However, the presence of CRAB features, high total protein, and lytic lesions shifted the focus toward a plasma cell disorder. Notably, her serum osmolality remained normal (S. osm 295), the hyponatremia was not true but pseudohyponatremia -in which sodium is falsely low but true plasma sodium concentration is normal, the non-aqueous portion of the plasma is increased in marked hyperproteinemia or hyperlipidemia. Measuring sodium by indirect ion-selective electrode measurements, which is used by most lab analyzers can lead to an incorrect measurement Discussion: Misinterpreting pseudohyponatremia as true hyponatremia could lead to unnecessary or harmful interventions (e.g., fluid restriction or hypertonic saline). This case emphasizes the importance of clinical correlation and serum osmolality measurement in such contexts. Recognizing pseudohyponatremia in the setting of paraproteinemia can prevent diagnostic delays and mismanagement. Prompt hematologic evaluation is crucial for early intervention and improved outcomes.