Multiple myeloma masquerading as severe seropositive rheumatoid arthritis with subcutaneous nodules and mononeuritis multiplex

Multiple myeloma can rarely mimic seronegative rheumatoid arthritis (RA). We report a 55‐year‐old woman who presented with longstanding deforming polyarthritis with extensive subcutaneous nodules, tenosynovitis, anti‐cyclic citrullinated peptide positivity and mononeuritis multiplex. Even though the clinical picture was consistent with seropositive RA, the absence of bone erosion or joint space narrowing on hand and knee radiographs led us to question the diagnosis of RA. Further investigation revealed a diagnosis of multiple myeloma with cutaneous amyloid deposits, based on serum immunofixation, bone marrow aspiration and biopsy of a subcutaneous nodule. The only clue to suspect myeloma from the basic investigations and clinical examination was mild hypercalcemia. This case serves to reiterate the need to maintain a heightened suspicion for other diagnoses even when RA appears most likely.