[Spurious hyperphosphatemia in multiple myeloma]

We present a 68-year-old woman with Ig A-K multiple myeloma and a laboratory report of extreme hyperphosphatemia (29 mg/dl), but no clinical manifestations attributable to such a high serum phosphorus. The hyperphosphatemia was demonstrated to be spurious and due to interference of monoclonal immunoglobulin with the phosphomolybdate calorimetric assay for phosphorus of some automated systems. In this case, after removal of serum proteins phosphorus values were normal. Normal values of phosphorus were also obtained with the phosphomolybdate calorimetric assay after reduction of paraprotein levels in this patient by chemotherapy. Knowledge of this phenomenon may obviate unnecessary testing and treatment. It may also suggest the presence of dysproteinemia.