Detecting interferon alpha in synovial fluid. Rheumatoid arthritis (RA) synovial fluid (SF), psoriatic arthritis (PsA) SF and osteoarthritis (OA) SF contain interferon alpha (IFN-α). The level of IFN-α in SF from RA patients (n = 18), PsA patients (n = 15) and OA patients (n = 8) was measured by ELISA. Squares indicate individual samples expressed on a log scale. -, median.