TNF and rheumatoid arthritis: Another study’s evaluation of the degree of clinical rheumatoid arthritis activity based on the concentrations of cytokines tumor necrosis factor-α, interleukins -12, -15, and -18 in serum and synovial fluid concluded that cytokines concentrations could be good indicators of the degree of the general activity of RA (Petrovic-Rackov, 2006).