A 58-year-old woman from Mumbai presents with recurrent infections, bone pain, and a 2-month history of progressive weakness. Investigations reveal: Hemoglobin 7.8 g/dL, WBC 3.2 × 10⁹/L, platelets 95 × 10⁹/L, serum creatinine 1.9 mg/dL, serum calcium 10.2 mg/dL, and serum albumin 2.3 g/dL. Serum protein electrophoresis shows an M-spike of 3.8 g/dL (IgA kappa). Bone marrow shows 38% plasma cells with cytoplasmic inclusions. Skeletal survey is normal with no lytic lesions. Serum free light chain ratio is 32:1. What is the most likely diagnosis?

Explanation

## Multiple Myeloma with High Tumor Burden **Key Point:** Multiple myeloma (MM) is a plasma cell dyscrasia characterized by clonal proliferation of plasma cells producing a monoclonal immunoglobulin. Diagnosis requires ≥10% clonal bone marrow plasma cells (or biopsy-proven plasmacytoma) PLUS evidence of end-organ damage (CRAB criteria) or myeloma-defining events (SLiM criteria). ### Why This Patient Has Active (Symptomatic) Multiple Myeloma | Feature | This Patient | Diagnostic Significance | |---------|-------------|------------------------| | **BM plasma cells** | 38% | ≥10% clonal plasma cells ✓ | | **M-spike** | 3.8 g/dL IgA kappa | Intact immunoglobulin M-protein ✓ | | **Anemia** | Hb 7.8 g/dL | CRAB criterion (C**R**AB) ✓ | | **Renal insufficiency** | Cr 1.9 mg/dL | CRAB criterion (CR**A**B) ✓ | | **Hypercalcemia** | Ca 10.2 mg/dL | Borderline; serum albumin 2.3 g/dL → corrected Ca = 10.2 + 0.8×(4−2.3) = **11.56 mg/dL** ✓ | | **Skeletal survey** | Normal | Lytic lesions absent, but not required if other CRAB met | | **FLC ratio** | 32:1 | Abnormal but <100:1; does NOT define light chain disease | **High-Yield (Harrison's 21e, Chapter 109):** The CRAB criteria for symptomatic MM are: **C**alcium elevation (corrected >11 mg/dL), **R**enal insufficiency (creatinine >2 mg/dL or CrCl <40 mL/min), **A**nemia (Hb <10 g/dL or >2 g/dL below normal), **B**one lesions (≥1 lytic lesion). This patient meets at least anemia and renal insufficiency (borderline), with corrected hypercalcemia — fulfilling active MM criteria. **Clinical Pearl:** The presence of an **intact IgA kappa M-spike of 3.8 g/dL** on SPEP is the critical distinguishing feature. Light chain disease (Bence Jones myeloma) is defined by production of **free light chains only** — there is NO intact immunoglobulin heavy chain, and therefore NO M-spike on SPEP. A 3.8 g/dL IgA kappa M-spike is definitively an intact immunoglobulin, ruling out light chain disease. The FLC ratio of 32:1, while abnormal, does not reach the >100:1 threshold used to define light chain-only myeloma. ### Why Not the Other Options? **Light chain disease (Bence Jones myeloma):** Requires absence of intact immunoglobulin on SPEP. This patient has a prominent 3.8 g/dL IgA kappa M-spike — intact heavy chain is present. FLC ratio of 32:1 is insufficient to diagnose light chain disease (threshold >100:1 per IMWG criteria). This option is **incorrect**. **Smoldering multiple myeloma (SMM):** Defined as serum M-protein ≥3 g/dL OR BM plasma cells 10–60% OR FLC ratio 100–1000:1, **without** CRAB criteria or myeloma-defining events. This patient has active end-organ damage (anemia Hb 7.8, renal insufficiency, corrected hypercalcemia), so SMM is excluded. **Waldenström macroglobulinemia:** Produces **IgM** monoclonal protein in the setting of lymphoplasmacytic lymphoma. This patient has IgA kappa — not IgM. Excluded. **Mnemonic:** **CRAB** = **C**alcium ↑, **R**enal failure, **A**nemia, **B**one lesions → any one criterion + clonal plasma cells = Active MM (Robbins 10e, Chapter 13).