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.