Summary

A Japanese woman aged 76 years with rheumatoid arthritis treated with prednisolone and tocilizumab presented with a 2-day history of redness and pain in her right thigh. She was hospitalised with a primary diagnosis of cellulitis and antimicrobial therapy was initiated. She had been stable until the fourth day of admission, when the swelling of her right thigh rapidly worsened and demonstrated purpura; she was subsequently unable to walk because of the pain. A diagnosis of necrotising soft tissue infection (NSTI) was made and extensive debridement was performed. Over the next 4 months, additional debridement was performed four times. Her condition improved significantly and she was able to walk later. Physicians should recognise that tocilizumab can mask systemic toxicities and inflammatory findings even in severe infections. To avoid delays in diagnosis and surgical intervention, clinicians should consider NSTIs when they encounter patients treated with tocilizumab, even if it mimics cellulitis.