By definition the monoclonal protein is 10% bone marrow plasma cells and/or >30 g/L monoclonal serum protein but no myeloma symptoms. It is a common disorder with a prevalence of 3%–4% in the Caucasian population over the age of 50, occurs more frequently in men than women, and is at least 2-fold more prevalent in Africans and African Americans compared to Caucasians. MGUS is usually detected as an incidental finding when protein electrophoresis is done as part of laboratory studies for a wide range of symptoms. Although there is significant overlap, the distinctions of MGUS, SMM, and MM are made because the diseases have different biology. Although these conditions do not manifest symptoms that can be attributed to the plasma cell clone or monoclonal protein, they are called “pre-malignant” disorders because virtually all cases of multiple myeloma (MM) are now considered to have been preceded by MGUS or SMM as many as 8 or more years earlier. Pre-malignant disorders include monoclonal gammopathies of undetermined significance (MGUS) and smoldering multiple myeloma (SMM). Laboratory testing requirements for those will be discussed in a separate chapter in this CCLM special issue. This concept has been described for the first time in 2006 and applies to conditions such as immunoglobulin light chain amyloidosis (AL). The low-tumor-burden diseases represent a group of monoclonal gammopathies in which there may not be a large clonal proliferation of plasma cells but in which the cell products cause pathology. MGUS, monoclonal gammopathy of undetermined significance SMM, smoldering multiple myeloma POEMS, polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes BHPW, benign hypergammaglobulinemia purpura of Waldenström’s. These disorders are often categorized as protein/low-tumor-burden diseases, premalignant disorders, and malignancies. Table 1 lists by decreasing frequency of diagnosis 39,929 monoclonal gammopathies that were detected in the Mayo Clinic practice between 19. Plasma cell proliferative diseases have different presentations, treatments, and outcomes. It is this diversity of structure and concentration that make this clonal marker so interesting and challenging. In addition, although some plasma cell proliferative disorders present with M-protein concentrations of grams per liter of serum, others have little or virtually no circulating M-protein. The M-proteins each have unique variable region sequences and the molecules may range from pentameric IgM (~900,000 Daltons) to monomeric free light chains (~24,000 Daltons). Unlike most serologic tumor markers, M-proteins are extremely diverse. The secreted proteins can be used as a diagnostic tool for the identification of the clone of plasma cells as well as a quantitative marker to follow the course of the disease and response to therapy. Because of this secreted monoclonal immunoglobulin, these diseases are also called monoclonal gammopathies. Immunoglobulins are produced by plasma cells, and clonal plasma cell proliferative diseases usually secrete a monoclonal immunoglobulin (M-protein) that can be used as a serologic “tumor” marker. Monoclonal gammopathies overview and categorization The panel currently recommended for diagnostic screening is serum protein electrophoresis, immunofixation electrophoresis, and free light chain quantitation. The choice of tests for inclusion in diagnostic and monitoring panels may need to be tailored to each patient, and examples are provided. In this review we discuss the various disease presentations and the use of various tests such as protein electrophoresis and immunofixation electrophoresis as well as immunoglobulin quantitation, free light chain quantitation, and heavy-light chain quantitation by immuno-nephelometry. Panels of tests have been recommended for sensitivity and efficiency. Because of these challenges, no single test can confidently diagnose or monitor all patients. The difficulties of using monoclonal proteins for diagnosing and monitoring multiple myeloma, however, stem from the diverse disease presentations and broad range of serum protein concentrations and molecular weights. The unique structure of each monoclonal protein makes them highly specific for each plasma cell clone. Monoclonal immunoglobulins are markers of plasma cell proliferative diseases and have been described as the first (and perhaps best) serological tumor marker.
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