Mproteins

M-proteins, also known as monoclonal proteins or M-proteins, are abnormal immunoglobulins produced by a single clone of plasma cells. They are typically detected in the blood or urine and are a hallmark of plasma cell dyscrasias.

M-proteins can be intact immunoglobulins that include both heavy and light chains (for example IgG with kappa or lambda light chains) or free light chains released by malignant or clonal plasma cells. In some conditions, such as light-chain–predominant disorders, the circulating protein consists mainly of free kappa or lambda light chains without the heavy chain component.

Detection and monitoring of M-proteins rely on laboratory techniques such as serum protein electrophoresis and immunofixation, which identify and characterize monoclonal bands. Quantitative assays measure the amount of the M-protein or the ratio of kappa to lambda free light chains. Monitoring over time helps assess disease activity, response to therapy, and minimal residual disease in conditions like multiple myeloma. The M-protein level can guide clinical decisions, but its absence does not rule out disease, and its presence must be interpreted with other clinical and laboratory findings.

Clinical significance: M-proteins are central to diagnosis, prognosis, and monitoring of plasma cell disorders. Their appearance, type, and quantity influence diagnostic categorization (for example, multiple myeloma, monoclonal gammopathy of undetermined significance, Waldenström macroglobulinemia, or light-chain amyloidosis) and treatment planning. In MGUS, the M-protein is present at low levels with a risk of progression to malignant disease, which increases with time and certain risk factors.