What is the diagnostic implication of a serum protein electrophoresis (SPEP) finding of an M-spike, and what confirms the monoclonal protein?

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A sharp M-spike on serum protein electrophoresis signals a monoclonal gammopathy produced by a single clone of plasma cells, rather than a diffuse, polyclonal increase. This finding raises suspicion for disorders such as MGUS, multiple myeloma, or Waldenström macroglobulinemia, and it indicates a monoclonal protein is present. To confirm the monoclonal nature and identify the exact type of immunoglobulin and light chain, immunofixation electrophoresis is used. It pinpoints the heavy-chain class and light-chain type (for example, IgG kappa), distinguishing a true monoclonal protein from a polyclonal pattern and guiding further diagnosis and management.