What is the significance of M protein in multiple myeloma?

M protein, or monoclonal protein, is a key marker in multiple myeloma. It is produced by abnormal plasma cells in the bone marrow and can be measured in blood or urine. Doctors rely on M protein levels to:

• Confirm a diagnosis of multiple myeloma or related plasma cell disorders
• Measure disease burden, showing how much cancer is present in the body
• Monitor treatment response, since falling M protein levels typically indicate therapy is working
• Detect relapse early, as rising levels may signal the cancer is returning

Because M protein tracks so closely with disease activity, it remains one of the most important tools for managing multiple myeloma over time.

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Reviewed for medical accuracy: 07/14/2026