A 62-year-old man from Delhi presents with persistent lower back pain for 3 months and progressive fatigue. On examination, he appears pale. Laboratory investigations reveal: hemoglobin 8.2 g/dL, serum calcium 11.8 mg/dL, serum creatinine 2.1 mg/dL, and serum albumin 2.8 g/dL. Serum protein electrophoresis shows an M-spike of 4.2 g/dL in the gamma region. Bone marrow biopsy reveals 35% plasma cells with abnormal morphology. Which of the following is the most likely diagnosis?

Explanation

## Diagnostic Approach to Plasma Cell Dyscrasias **Key Point:** Multiple myeloma is diagnosed when ≥10% clonal bone marrow plasma cells OR ≥30% clonal bone marrow plasma cells (regardless of CRAB criteria) are present, along with myeloma-defining events. ### Clinical Features Present in This Case | Feature | Finding | Significance | |---------|---------|---------------| | Bone marrow plasma cells | 35% | Meets diagnostic threshold (≥10%) | | M-spike | 4.2 g/dL | Significant monoclonal protein | | Hypercalcemia | 11.8 mg/dL | CRAB criterion (C = Calcium) | | Renal impairment | Cr 2.1 mg/dL | CRAB criterion (R = Renal) | | Anemia | Hb 8.2 g/dL | CRAB criterion (A = Anemia) | | Bone pain | Lower back pain | CRAB criterion (B = Bone lesions) | | Hypoalbuminemia | 2.8 g/dL | Reflects disease burden | **High-Yield:** The patient meets BOTH diagnostic criteria: 1. ≥10% clonal bone marrow plasma cells (35% present) 2. ≥1 CRAB criterion (has all four: hypercalcemia, renal failure, anemia, bone disease) ### Why This Is Multiple Myeloma and Not MGUS **Clinical Pearl:** MGUS has <10% bone marrow plasma cells, <3 g/dL M-spike, and NO end-organ damage. This patient has 35% plasma cells + CRAB criteria, making myeloma certain. ### Diagnostic Algorithm ```mermaid flowchart TD A[Monoclonal protein detected]:::outcome --> B{Bone marrow plasma cells?}:::decision B -->|<10%| C{CRAB criteria present?}:::decision B -->|≥10%| D[Myeloma-defining event present?]:::decision C -->|No| E[MGUS]:::outcome C -->|Yes| F[Smoldering myeloma]:::outcome D -->|Yes| G[Multiple Myeloma]:::action D -->|No| H[Asymptomatic myeloma]:::outcome A --> I{IgM type?}:::decision I -->|Yes + lymphadenopathy| J[Waldenström macroglobulinemia]:::outcome I -->|No| B ``` **Mnemonic: CRAB criteria** — **C**alcium elevation, **R**enal insufficiency, **A**nemia, **B**one lesions **Key Point:** Modern diagnostic criteria (IMWG 2014) also include myeloma-defining events: - ≥60% clonal bone marrow plasma cells - Involved/uninvolved free light chain ratio ≥100 - >1 focal lesion on MRI This patient clearly has multiple myeloma based on both classical CRAB criteria and elevated plasma cell percentage.