Spuriously high phosphate level which is promptly resolved after plasmapheresis in a patient with multiple myeloma

We present a 57 year old female patient with IgG multiple myeloma and marked hyperphosphatemia. The patient had no clinical symptoms related to hyperphosphatemia. Serum inorganic phosphate measurements were repeated on sulfosalicylic acid deproteinized serum samples, yielding normal phosphate levels. We realized that this hyperphosphatemia was spurious because of high paraprotein levels. Afterwards, therapeutic plasma exchange (TPE) was administered due to hyperviscosity and bleeding tendency. After the administration of TPE, serum phosphate was reduced to normal level. Therapeutic plasma exchange resulted in steady-state serum phosphate levels during the three months follow up period. We concluded that high phosphate levels must be confirmed by measuring in deproteinized serum samples in multiple myeloma patients in order to distinguish pseudohyperphosphatemia from the true ones. In addition, TPE effectively reduces the spuriously elevated phosphate levels possibly by removing paraproteins.