## Distinguishing Multiple Myeloma from Waldenström Macroglobulinemia ### Key Discriminating Feature **Key Point:** The immunoglobulin isotype is the most reliable discriminator between these two paraproteinemias. Multiple myeloma is predominantly IgG (50–60%) or IgA (20–25%), whereas Waldenström macroglobulinemia is **always IgM**. ### Comparison Table | Feature | Multiple Myeloma | Waldenström Macroglobulinemia | | --- | --- | --- | | **Monoclonal protein isotype** | IgG, IgA, IgD, IgE (rarely IgM) | **IgM (100%)** | | **Lytic bone lesions** | Present (CRAB criteria) | Absent (no bone involvement) | | **Bone marrow plasma cells** | >30% (diagnostic) | <30%, mostly lymphoplasmacytoid cells | | **Hyperviscosity** | Rare (IgG/IgA less viscous) | Common (IgM is pentameric, highly viscous) | | **Organomegaly** | Absent | Common (splenomegaly, hepatomegaly) | | **Lytic lesions vs. osteoporosis** | Punched-out lytic lesions | Generalized osteoporosis | ### Clinical Pearl **Clinical Pearl:** IgM paraproteinemia in a patient with bone pain and lytic lesions is a **red flag for multiple myeloma, NOT Waldenström**. Conversely, IgM paraproteinemia with hyperviscosity and NO lytic lesions points to Waldenström. ### High-Yield Mnemonic **Mnemonic:** **CRAB** = Calcium elevation, Renal insufficiency, Anemia, Bone lesions — these are the diagnostic criteria for symptomatic myeloma. Waldenström has none of these; instead it has **HVS** = Hyperviscosity, Villous lymphocytes, Splenomegaly. ### Why This Matters Both conditions present with monoclonal paraproteinemia and bone marrow infiltration, but the **isotype of the monoclonal protein** is the single most reliable discriminator: - **IgM → Waldenström** (by definition) - **IgG/IgA/IgD/IgE → Multiple Myeloma** (IgM in myeloma is <1%) This distinction is critical because treatment, prognosis, and complications differ significantly.
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