Serratia Marcescens Septic Arthritis in a Rheumatoid Patient

Case: A 51-year-old White woman with rheumatoid arthritis presented to the ED with right knee pain. She was initially discharged with steroids and anti-inflammatory medicines. On her third ED visit, knee arthrocentesis showed synovial white blood count (WBC) of 18,800 E9/L, 90% neutrophils, calcium pyrophosphate deposition (CPPD) crystals, and Serratia marcescens in cultures. Arthroscopic irrigation and debridement were performed. She was given IV antibiotics and discharged on postoperative day 5. This case expands the diagnostic challenge posed by dual diagnosis—CPPD masking or complicating the recognition of septic arthritis.