Plasma cell dyscrasias: classification, clinical and laboratory characteristics, and differential diagnosis.

Plasma cell dyscrasias form a heterogeneous group of diseases characterized by the expansion of the number of monoclonal bone marrow plasma cells that produce monoclonal immunoglobulins. Sensitive electrophoretic methods have shown that the incidence of these diseases is as high as 5% in adult individuals. Thus, the majority of cases should be considered to be a normal phenomenon. A few transform into neoplastic diseases, plasma cells becoming responsible for lytic bone lesions, the hallmark of MM. The distinction of benign and malignant forms is frequently difficult at presentation. We can easily recognize solitary myeloma, overt myeloma and plasma cell leukaemia, which require immediate chemotherapy. Therapy could be safely withheld in all the remaining forms, which require only follow-up. Thus, we suggest that plasma cell dyscrasias should be classified simply into two main groups according to the need of immediate chemotherapy. The appearance of new bone lesions and the increase of the M-component level remain the only two criteria that define malignant transformation. Several clinical and laboratory prognostic parameters indicate the risk of transformation, and hence how close the follow-up of the patient should be. Parameters related to the expansion of the plasma cell clone (percentage of bone marrow plasma cells, M-component level, lytic bone lesions and beta 2-microglobulin) are not always very low and very high in the benign and malignant forms, respectively, and frequently overlap in patients with intermediate plasma cell expansions. On the contrary, all parameters related to the intrinsic malignancy of the plasma cells (plasma cell LI, Karyotypic abnormalities and molecular alterations) have, by definition, to be normal in the benign forms. MRI is a new tool that may, early on, reveal lytic bone lesions undetectable by conventional radiography.