In multiple myeloma there is a clonal proliferation of plasma cells, usually involving the bone marrow. The differential diagnosis includes reactive states or the condition termed monoclonal gammopathy of unknown significance (MGUS). After immunostaining the bone marrow with antibodies to kappa and to lambda light chains, the plasma cells which positively stain can be counted. The relative number of plasma cells staining for kappa vs lambda light chains can help in diagnosis.
