Diagnostic Dilemma of Anemia and Oligoarthritis After Tumor Necrosis Factor Inhibitor Therapy in Spondyloarthritis: Flare, Infection, or Paradoxical Reaction?

In patients with axial spondyloarthritis (axSpA), inadequate disease control may necessitate escalation to immunosuppressive therapy.However, subsequent clinical worsening can create a diagnostic dilemma, particularly when disease activity, treatment-related paradoxical reactions, and infection present with overlapping features.We report a 33-year-old man with axSpA receiving targeted synthetic diseasemodifying antirheumatic drug (DMARD) therapy who developed peripheral large-joint oligoarthritis.Suspected inadequate disease control led to treatment escalation with a tumor necrosis factor inhibitor (TNFi).He subsequently developed fever, anemia, and worsening arthritis.Infectious workup and synovial fluid analysis were inconclusive.Bone marrow examination showed pure red cell aplasia (PRCA), and polymerase chain reaction (PCR) testing confirmed a high parvovirus B19 viral load.He improved following treatment with intravenous immunoglobulin (IVIg).This case highlights the importance of considering parvovirus B19 infection in patients with inflammatory arthritis and unexplained cytopenias, as early diagnosis can prevent unnecessary escalation of immunosuppressive therapy.