In monoclonal gammopathy evaluation, which test confirms the type of monoclonal protein after an M-spike is detected?

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When an M-spike is seen, pinpointing the exact monoclonal protein type is essential for diagnosis and management. Immunofixation uses antisera specific for immunoglobulin heavy chains (IgG, IgA, IgM, etc.) and light chains (kappa, lambda) to identify the class and light-chain type of the monoclonal protein. The pattern shows a single, discrete band in the corresponding heavy-light chain lanes, confirming the M-protein’s isotype. This makes it the confirmatory test for typing the monoclonal protein.

Other tests don’t provide this precise typing. Western blot isn’t routinely used for monoclonal protein typing in most labs and is less practical for this purpose; a CBC with differential counts plasma cells but doesn’t identify the immunoglobulin class; urinalysis detects free light chains in urine but doesn’t type the serum M-protein.