C reactive protein may not be reliable as a marker of severe bacterial infection in patients receiving tocilizumab.

This is a case of a 65-year-old man with seropositive erosive rheumatoid arthritis (RA), well controlled on methotrexate, sulfasalazine, low-dose prednisolone and monthly infusions of tocilizumab. He presented with a 3-week history of pain and swelling in his left knee, gradually increasing in severity with an inability to bear weight. He was systemically well with normal vital signs. Examination confirmed an effusion and aspiration was turbid in appearance. C reactive protein (CRP) was normal. He was treated empirically with antibiotics. Synovial fluid and blood cultures confirmed Staphylococcus aureus infection. He completed a 6 weeks course of antibiotics with complete resolution of symptoms. Throughout the treatment his CRP remained normal which is likely to have been the result of prior treatment with tocilizumab.