Pseudohyperphosphatemia in a patient with multiple myeloma

Hyperphosphatemia, in patients with multiple myeloma, is usually associated with severely reduced renal function. Paraproteins can interfere with the measurement of serum inorganic phosphate when certain types of automatic methods are employed. We present the case of a 44-year-old male admitted to our hospital with severe back pain and an osteolytic lesion of the first lumbar vertebra. He was diagnosed with multiple myeloma based on serum protein electrophoresis and bone marrow biopsy results. An initial serum phosphate level of 12.5 mg/dl, (normal 2.5-5 mg/dl), in the setting of normal kidney function, did not improve after treatment with oral phosphate binders. The inorganic phosphate determinations were repeated on sulfosalicylic acid deproteinized serum samples, yielding normal range phosphate levels. This case reiterates the importance of using deproteinized serum samples for phosphate measurements in patients with multiple myeloma, normal renal function and elevated phosphate levels, before initiating therapy with phosphate binders.