## Distinguishing Multiple Myeloma from Waldenström Macroglobulinemia ### Key Discriminating Feature **Key Point:** The type of monoclonal immunoglobulin produced is the most reliable single discriminator between these two plasma cell dyscrasias. ### Comparative Table | Feature | Multiple Myeloma | Waldenström Macroglobulinemia | |---------|------------------|-------------------------------| | **Paraprotein type** | IgG (50%), IgA (25%), IgD (2%), Light chain (20%) | **IgM (>95%)** | | **Bone lesions** | Lytic (CRAB criteria) | Rare or absent | | **Hyperviscosity** | Uncommon | Common (IgM is pentameric) | | **Serum viscosity** | Normal or mildly elevated | Markedly elevated | | **BM infiltration** | >30% plasma cells | <30% lymphoplasmacytic cells | | **Organomegaly** | Absent | Present (splenomegaly, hepatomegaly) | ### Pathophysiologic Basis **High-Yield:** IgM is a pentameric immunoglobulin with high molecular weight (~900 kDa), making it the defining feature of Waldenström disease. Multiple myeloma produces IgG or IgA (monomeric or dimeric), which do not cause hyperviscosity despite higher serum levels. ### Clinical Pearl **Clinical Pearl:** A patient with monoclonal IgM paraprotein and hyperviscosity syndrome almost always has Waldenström macroglobulinemia, not myeloma. Conversely, lytic bone lesions (CRAB: Calcium elevation, Renal insufficiency, Anemia, Bone lesions) are hallmark of myeloma and absent in Waldenström. ### Why This Matters **Mnemonic:** **IgM = Waldenström** (both start with W in concept: IgM is the pentameric "W-shaped" immunoglobulin). Multiple myeloma = IgG, IgA, or light chain. **Tip:** On NEET PG, when comparing myeloma vs. Waldenström, always think "IgM = Waldenström" first. If the stem mentions IgM paraprotein, the answer is Waldenström until proven otherwise.
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