Hypophosphatemia associated with paraproteinemia: a case report and review of the literature.

The differential diagnosis for hypophosphatemia is long, and involves complex, overlapping physiological systems. Practitioners are often guilty, however, of simply supplementing phosphate without fully investigating the etiology of the problem. The purpose of this case presentation is to illustrate a case of spurious hypophosphatemia that initially led to unnecessary phosphate replacement in a woman with undiagnosed multiple myeloma. An 85-year-old African American woman was admitted to the hospital for congestive heart failure exacerbation. The patient was incidentally found to be profoundly hypophosphatemic and was also diagnosed with multiple myeloma at this hospitalization. Normal phosphorus levels were difficult to maintain despite aggressive replacement. A serum sample initially reported to have an abnormally low phosphorus concentration on the Beckman CX7 analyzer was reanalyzed with the Kodak Ektachem 700 system, revealing the phosphorus concentration to be towards the higher limit of the normal range on the same sample. We conclude that clinicians should proceed with caution before aggressively treating abnormal phosphorus levels in patients with known paraproteinemia. Conversely, unexplained phosphorus abnormalities should bring disorders associated with paraproteinemia, such as multiple myeloma, into the differential diagnosis. Knowledge of how various phosphorus assays are affected by paraproteins is essential to guiding diagnosis and treatment. We also review mechanisms of reported interference with common assays.