Background: Spuriously low levels of plasma phosphate have been reported previously in patients with multiple myeloma and polyclonal gammopathy. We report 2 cases of spurious hypophosphatemia in patients with elevated concentrations of serum monoclonal immunoglobulins, 1 of whom had monoclonal gammopathy of undetermined significance and the other multiple myeloma.
Methods: Plasma phosphate concentrations were measured using nondeproteinized and deproteinized plasma samples from patients with monoclonal gammopathies.
Results: In 2 patients with monoclonal gammopathy, the levels of plasma inorganic phosphate were reported as <1.0 mg/dL when the phosphate concentration was determined using an analyzer that employs nondeproteinized plasma. When the samples were reanalyzed using a laboratory method that removes serum proteins, normal or elevated concentrations of phosphate were found. Plasma levels of phosphate in 4 other patients with monoclonal gammopathy were normal by both methods.
Conclusions: These data confirm previous reports that spurious hypophosphatemia occurs in some patients with increased levels of serum monoclonal immunoglobulins when laboratory methods using nondeproteinized samples are employed. The occurrence of unusually low plasma phosphate concentrations in patients without symptoms or clinically apparent causes of hypophosphatemia should alert physicians to search for monoclonal gammopathy.