Spurious hypophosphatemia and hyperphosphatemia in a patient with multiple myeloma

Abstract

Patients with multiple myeloma may present either hypophosphatemia as a consequence of proximal tubular dysfunction or hyperphosphatemia in the setting of renal failure. Nonetheless, spurious hypophosphatemia and hyperphosphatemia due to paraprotein-interfering serum phosphate assays have been occasionally described. A 64 year-old female with multiple myeloma with phosphatemia within the normal range at the time of diagnosis developed severe hypophosphatemia (<0.5 mg/dl). Potassium phosphate was administered intravenously and withdrew 8 hours later when serum phosphate levels normalized (3.8 mg/dl). Some days later serum phosphate was > 16 mg/dl. A diluted blood sample displayed a real value of 3.8 mg/dl. Two months later, a laboratory control showed hypophosphatemia < 0.5 mg/dl again but when the sample was diluted the phosphate level was 3.5 mg/dl. In patients with paraproteinemia and hyper or hypophosphatemia we must consider interference in the determination of phosphate. On the other hand when severe hypophosphatemia or hyperphosphatemia of unclear origin is present multiple myloma should be ruled-out. This is the first description of spurious hypophosphatemia and hyperphosphatemia attributable to reading-interfering effect of paraproteins occurring in a single individual.