Hyperphosphatemia in multiple myeloma due to a phosphate-binding immunoglobulin

Area of Science:

• Nephrology
• Clinical Biochemistry
• Immunology

Background:

• Hyperphosphatemia (HP) is commonly associated with conditions like hypoparathyroidism, renal failure, and tumor lysis syndrome.
• Paraproteinemias, particularly multiple myeloma, can sometimes interfere with laboratory assays, leading to spurious results.

Observation:

• An 86-year-old woman presented with significantly elevated serum phosphate levels (4.75 mmol/l) despite normal calcium, BUN, creatinine, and PTH.
• Elevated total protein and subsequent diagnosis of immunoglobulin G (IgG) multiple myeloma were noted.
• Subnormal 1,25-dihydroxyvitamin D levels (<12 pmol/l) were observed.

Findings:

• In vitro studies confirmed that the patient's IgG paraprotein directly bound inorganic phosphate.
• This binding is distinct from assay interference, suggesting a physiologically relevant mechanism for HP.
• This case documents actual phosphate binding by an IgG paraprotein.

Implications:

• This finding suggests that phosphate-binding paraproteins can cause true hyperphosphatemia with potential clinical significance.
• The observed decrease in 1,25-dihydroxyvitamin D may be a consequence of this phosphate dysregulation.
• Highlights the importance of considering paraprotein-associated phosphate binding in unexplained hyperphosphatemia, especially in myeloma patients.