These abnormalities are similar but not identical to those detected in sPCL while the abnormalities detected in sPCL more closely resemble those detected in multiple myeloma than do those of pPCL: the genetic data support the clinical data in suggesting that sPCL and pPCL are distinct diseases with sPCL among the two PCLs being more closely related to multiple myeloma. Examination of plasma cell immunophenotype by measuring certain of their cell surface antigens, particularly Cluster of differentiation. CD markers on plasma cells from patients with pPCL differ from those taken form multiple myeloma or sPCL patients. For example: pPCL plasma cells more often express CD20 antigen, which is considered important in anchoring plasma cells to the bone marrow stroma, than do those on plasma cells taken from myeloma patients (50% vs. 17%); pPCL plasma cells often lack CD56 antigen which is present on the majority of plasma cells taken form multiple myeloma patients; and pPCL plasma cells more frequently express CD28 than do sPCL plasma cells. Thus, immunophenotyping supports that notion that multiple myeloma, sPCL, and pPCL show critically important fundamental differences that may explain their different clinical presentations, courses, responses to therapy, and prognoses. Diagnosis
The International Myeloma Working Group has defined the diagnostic criteria for plasma cell leukemia as the presence in blood of >2x109 plasma cells per liter or, alternatively, >20% of nucleated blood cells being plasma cells.
thrombocytopenia, anemia, and/or, rarely, leukopenia), central nervous system defects, and peripheral neuropathies due to the invasion of these tissues by plasma cells and/or the deposition of their circulating monoclonal immunoglobulin in them. Compared to multiple myeloma patients, pPCL patients also: exhibit 1) high rates of developing a hypercalcemic crisis, i.e. a potentially life-threatening episode of high ionic calcium (Ca2+) levels in the blood due to excess bone re-absorption and/or renal failure; b)  higher levels of serum lactate dehydrogenase and Beta-2 microglobulin; and c) lower rates of bone but higher rates of soft tissue plasma cell tumors termed plasmacytomas. Secondary PCL
Secondary PCL (sPCL) is diagnosed in 1-4% of patients known to have had multiple myeloma for a median time of ~21 months. It is the terminal phase of these patients myeloma disease. sPCL patients typically are highly symptomatic due to extensive disease with malignant plasma cell infiltrations in, and failures of, not only the bone marrow but also other organs. They have failed or broken through one or more treatment regimens and therefore may also show some of the toxic effects of these treatments. Cause
PCL is caused by the development of an excessively high number of genetic abnormalities in plasma cells or, more particularly, their precursor B cells and plasmablasts (see plasma cells).