Hospitalization and Diagnosis
Both the bacterial culture of synovial fluid and blood culture showed no growth. Septic arthritis was less likely. Connective tissue disease survey showed only equivocal result of ANA (1:80) and no specific autoantibodies or clinical features resembling of autoimmune disease (ex. Skin rash, Raynaud phenomenon, myositis, sclerodactly) were found. Seronegative (RF, anti-CCP -) rheumatoid arthritis (RA) and adult onset still's disease (AOSD) were still two of the key differential diagnosis regarding polyarthritis. Nonetheless, the large joint involvement in our patient may be a useful clue to question about the diagnosis of RA. Furthermore, absence of typical evanescent skin rash, sore throat or highly elevated ferritin somewhat excluded the diagnosis of AOSD. At last, the synovial fluid analysis by polarizing light microscopy showed positive calcium pyrophosphate (CPP) crystal (Picuture13), which was diagnostic of calcium pyrophosphate deposition (CPPD) disease. The x-ray of knees and wrist also demonstrated  chondrocalcinosis, supporting the diagnosis. Following steroid, colchicne and NSAID, the arthritis and fever subsided rapidly and completely (Picuture 14).

Picuture14. Treatment response