Pseudohyperphosphatemia related to a monoclonal immunoglobulin in a patient with plasma cell leukemia: an analytical interference to be aware of

We report a case of pseudohyperphosphatemia in a patient with multiple myeloma that progressed to plasma cell leukaemia. This pseudohyperphosphatemia was caused by an analytical interference between monoclonal immunoglobulin and ammonium molybdate, a reagent used for measuring inorganic phosphate. Accurate measurement of blood inorganic phosphate levels requires prior protein precipitation to eliminate assay interference. Extrarenal purification treatments helped reduce the monoclonal immunoglobulin concentration and eliminated the interference during phosphate measurement. Pseudohyperphosphatemia is a rare event that can lead to inappropriate and harmful medical management for affected patients; thus, prompt recognition by clinical biologists is crucial to avoid misdiagnosis and inappropriate interventions. This case report also provides an opportunity to do a brief review of the literature and to remind some recommendations and actions to take into consideration in the presence of discrepancies between the clinic and the biology.