Lesson of the month 1: Septic arthritis with normal acute phase reactants and white cell count in a patient receiving tocilizumab.

Septic arthritis represents 8-27% of cases of monoarthritis presenting to the emergency department. Tocilizumab is an interleukin-6 blocking monoclonal antibody with the mechanistic potential to interfere with the body's normal inflammatory response to an infectious insult. We present a case of septic arthritis with a normal white cell count, C-reactive protein, erythrocyte sedimentation rate and fibrinogen in a patient treated with tocilizumab. © Royal College of Physicians 2017. All rights reserved.