## Bone Marrow Histology in Multiple Myeloma **Key Point:** The hallmark histologic finding in multiple myeloma is increased bone marrow plasma cells (>10% of nucleated cells), often with abnormal morphology including immature features, binucleate forms, and increased mitotic activity. ### Diagnostic Criteria for Multiple Myeloma (IMWG 2014) | Criterion | Threshold | Notes | | --- | --- | --- | | **Bone marrow plasma cells** | ≥10% | Hallmark finding; abnormal morphology supports diagnosis | | **Serum monoclonal protein** | ≥3 g/dL | IgG, IgA, IgD, or IgM | | **Urine monoclonal protein** | ≥500 mg/24 h | Bence Jones proteinuria | | **CRAB criteria** | ≥1 present | Calcium elevation, Renal insufficiency, Anemia, Bone lesions | | **Clonality** | Confirmed | Serum/urine electrophoresis + immunofixation | ### Morphologic Features of Abnormal Plasma Cells in MM - **Immature cytology:** Irregular nuclear membranes, coarse chromatin, prominent nucleoli - **Increased mitotic activity:** Abnormally high mitotic index - **Binucleate/multinucleate forms:** Dysplastic features - **Plasmacytosis:** Sheets or clusters replacing normal marrow elements - **Cytoplasmic inclusions:** Russell bodies (immunoglobulin aggregates), Dutcher bodies (nuclear immunoglobulin) **High-Yield:** A bone marrow plasma cell percentage ≥10% is **required** for the diagnosis of multiple myeloma. This is one of the three pillars of the diagnostic triad (along with serum/urine monoclonal protein and CRAB/SLiM criteria). **Clinical Pearl:** The morphologic abnormality of plasma cells in MM can be graded by the **Matutes score** or similar systems; more immature morphology correlates with higher proliferation index and worse prognosis. **Mnemonic:** **"10% Plasma = MM Diagnosis"** — Remember that ≥10% clonal bone marrow plasma cells is the morphologic threshold for MM diagnosis.
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