What is the hallmark serum protein electrophoresis finding in multiple myeloma?

• A. Monoclonal (M) spike
• B. Polyclonal hypergammaglobulinemia
• C. Delta chain fragmentation
• D. Hypogammaglobulinemia

Answer: (A)

A monoclonal spike on serum protein electrophoresis indicates a single clone of plasma cells producing one type of immunoglobulin. In multiple myeloma, malignant plasma cells secrete a uniform immunoglobulin, creating a sharp, narrow band (the M-spike) usually in the gamma region. This clonal protein distinguishes it from a polyclonal gammopathy, which shows a broad, diffuse increase in many immunoglobulins. Some myelomas may secrete only light chains, which can be detected as free light chains in serum or Bence Jones proteins in urine, but the defining feature remains the monoclonal spike from a single immunoglobulin type.