A 58-year-old man from Mumbai presents with bone pain and anemia. Serum protein electrophoresis shows an M-spike of 4 g/dL (IgG-kappa). Bone marrow biopsy reveals 35% plasma cells. Which finding most reliably distinguishes this patient's condition (multiple myeloma) from monoclonal gammopathy of undetermined significance (MGUS)?

## Multiple Myeloma vs. MGUS: Diagnostic Discrimination ### Diagnostic Criteria Comparison | Criterion | MGUS | Multiple Myeloma | |-----------|------|------------------| | **M-spike** | <3 g/dL | ≥3 g/dL (or ≥1.5 g/dL if IgM) | | **BM plasma cells** | <10% | ≥10% (or ≥30% in this case) | | **CRAB/SLiM criteria** | Absent | ≥1 present | | **Lytic lesions** | Absent | Present (CRAB: **B**one lesions) | | **Renal dysfunction** | Absent | Present (CRAB: **R**enal insufficiency) | | **Anemia** | Absent | Present (CRAB: **A**nemia) | | **Hypercalcemia** | Absent | Present (CRAB: **C**alcium elevation) | | **SLiM criteria** | — | ≥60% BM plasma cells, involved/uninvolved FLC ratio ≥100, or ≥2 focal lesions on MRI | ### Key Point: CRAB and SLiM Criteria **Key Point:** The International Myeloma Working Group (IMWG) defines myeloma by the presence of ≥10% clonal bone marrow plasma cells AND ≥1 CRAB criterion OR ≥1 SLiM criterion. MGUS has none of these. **High-Yield:** **CRAB = Ca (Calcium), R (Renal), A (Anemia), B (Bone lesions)**. Any ONE of these in the presence of clonal plasma cells = myeloma, not MGUS. ### Clinical Pearl **Clinical Pearl:** A patient with 35% plasma cells and an M-spike >3 g/dL is NOT automatically diagnosed with myeloma unless CRAB or SLiM criteria are met. However, in this case, the presence of bone pain (suggesting lytic lesions) and anemia fulfills CRAB criteria, making this myeloma. ### Why This Case is Myeloma The patient has: 1. ≥10% clonal plasma cells (35%) ✓ 2. CRAB criteria: Anemia + Bone lesions (implied by bone pain) ✓ Therefore, this is **multiple myeloma**, not MGUS. ### Mnemonic **Mnemonic:** **CRAB** = **C**alcium, **R**enal, **A**nemia, **B**one. If you see any ONE of these + clonal plasma cells, it's myeloma. MGUS has NONE of these. **Tip:** On NEET PG, when distinguishing myeloma from MGUS, always check for CRAB or SLiM criteria first. The M-spike level and BM % alone are insufficient without end-organ damage.