Plasma cell labeling index.

Multiple myeloma is characterized by proliferation of monoclonal plasma cells (PCs), mostly in the bone marrow. The proliferative rate of the malignant plasma cell is an important determinant of the disease biology and can be measured as the percentage of PCs in the S-phase of the cell cycle. This percentage, or PC labeling index, can be measured using a slide-based immunofluorescence method using an antibody against 5-bromo-2'-deoxyuridine, which is actively incorporated by DNA of the dividing PCs. This technique, which can be performed using bone marrow or peripheral blood specimens, also utilizes concurrent cytoplasmic staining against immunoglobulin as well as kappa and lambda light chains. Employment of cytoplasmic immunoglobulin staining allows more specific identification of PCs as well as confirmation of the monoclonal nature of the PC population. The staining procedure, which can be done manually or using an automated stainer, as well as the process of reading and interpreting these slides is described in detail. The bone marrow peripheral blood labeling index is an important clinical test, providing valuable diagnostic and prognostic information.