Presence of crystals is not an evidence of absence of infection.

Acute monoarthritis is one of the most common rheumatologic presentations. However, it is clinically difficult to distinguish between an inflamed joint due to crystal-induced arthritis and an inflamed joint due to septic arthritis. Arthrocentesis and synovial fluid analysis are used to differentiate between these 2 conditions. The presence of crystals and positive synovial fluid culture confirm the diagnosis of crystal-induced arthritis and septic arthritis, respectively. Although uncommon, these 2 arthritides can coexist, and presence of crystal does not exclude bacterial arthritis. We reported a case of 85-year-old woman whose synovial fluid contained crystals and was initially diagnosed with crystal-induced arthritis. However, her joint fluid culture subsequently grew Staphylococcus aureus, and she was treated with arthroscopic debridement and antibiotics.