In multiple myeloma, what does M-protein indicate?

• A. Monoclonal immunoglobulin detected in serum and urine
• B. Polyclonal increase in immunoglobulins
• C. Absence of immunoglobulins
• D. IgM only

Answer: (A)

In multiple myeloma, M-protein is a monoclonal immunoglobulin produced by the malignant plasma cell clone. It shows up as a sharp M-spike on serum protein electrophoresis and can be detected in the urine as Bence Jones proteins (free light chains). This monoclonal protein reflects a single clone of immunoglobulin-producing cells, which distinguishes it from the polyclonal immunoglobulin increases seen with infections or inflammatory states. It isn’t an absence of immunoglobulins, and while some diseases involve IgM monoclonal proteins, multiple myeloma most often involves IgG or IgA (or light chains), not IgM alone. So M-protein indicates monoclonal immunoglobulin detected in serum and urine.