A 58-year-old man with a 10-year history of rheumatoid arthritis presents with nephrotic syndrome and hepatosplenomegaly. Renal biopsy shows amyloid deposition. Which finding would most reliably distinguish his amyloidosis from a patient with multiple myeloma presenting with similar renal amyloidosis?

Explanation

## AA vs AL Amyloidosis in Renal Disease ### Clinical Scenario Analysis **Key Point:** This patient has secondary (AA) amyloidosis due to chronic inflammatory arthritis, not primary (AL) amyloidosis from plasma cell dyscrasia. The distinguishing feature is the **protein composition** of the amyloid, not the clinical presentation or bone marrow findings. ### Pathophysiology Comparison | Parameter | AA Amyloidosis (This Patient) | AL Amyloidosis (Multiple Myeloma) | |-----------|---|---| | **Trigger** | Chronic inflammation (RA, TB, osteomyelitis) | Plasma cell dyscrasia (monoclonal) | | **Amyloid protein** | Serum amyloid A (hepatic acute-phase reactant) | Immunoglobulin light chains (κ or λ) | | **Bone marrow** | Normal or reactive plasmacytosis | Clonal plasma cell infiltration (>10%) | | **Serum protein electrophoresis** | Normal or polyclonal pattern | Monoclonal spike (M-protein) | | **Immunohistochemistry** | AA protein positive; light chains negative | κ or λ light chain positive; AA negative | | **Organ involvement** | Kidneys >> liver, spleen | Heart, nerves, kidneys | ### Why Immunohistochemistry is the Discriminator **High-Yield:** Immunohistochemistry (or immunofluorescence) of the renal biopsy showing **AA protein positivity** definitively identifies secondary amyloidosis from chronic inflammation. In contrast, AL amyloidosis shows light chain (κ or λ) positivity. **Clinical Pearl:** A patient with RA and amyloidosis almost always has AA amyloidosis (secondary to chronic inflammation), not AL. However, a patient with multiple myeloma and amyloidosis has AL amyloidosis (from monoclonal light chains). The **tissue protein composition** is the gold standard discriminator. **Mnemonic:** **AA = Arthritis/Acute-phase**; **AL = Amyloid Light-chain**. ### Why Other Options Fail as Discriminators **Warning:** A monoclonal spike on SPEP can occur in both AL amyloidosis (from the plasma cell clone) and in some cases of AA amyloidosis (as an incidental finding or from a separate monoclonal gammopathy). It is not a reliable discriminator. - **Glomerular vs. vascular deposition:** Both AL and AA can deposit in glomeruli; this is not a consistent discriminator. - **Bone marrow findings:** While AL typically shows clonal plasma cells and AA shows reactive changes, bone marrow examination is not as direct or reliable as tissue immunohistochemistry. Additionally, some patients with AA may have reactive plasmacytosis that could be misinterpreted.